and detect leaked temporary files and mount entries after each test run.

Background

Currently we have several testing-related problems that are
all connected to /tmp and similar directories:

Problem 1: many tests create temporary files for each run. Usually
tests are careful to remove them on teardown, but due to bugs, many kind
of tests, test processes being hard-killed (SIGKILL, or SIGSEGV) and
other reasons, in practice this cleanup does not work 100% reliably and
there is steady growth of files leaked on /tmp on testnodes.

Problem 2: due to using shared /tmp and /dev/shm, the isolation in
between different test runs of potentially different users is not
strong. For example @jerome reports that due to leakage of faketime's
shared segments separate test runs affect each other and fail:
https://erp5.nexedi.net/bug_module/20211125-1C8FE17

Problem 3: many tests depend on /tmp being a tmpfs instance. This are for
example wendelin.core tests which are intensively writing to database,
and, if /tmp is resided on disk, timeout due to disk IO stalls in fsync
on every commit. The stalls are as much as >30s and lead to ~2.5x overall
slowdown for test runs. However the main problem is spike of increased
latency which, with close to 100% probability, always render some test
as missing its deadline. This topic is covered in
https://erp5.com/group_section/forum/Using-tmpfs-for--tmp-on-testnodes-JTocCtJjOd

--------

There are many ways to try to address each problem separately, but they
all come with limitations and drawbacks. We discussed things with @tomo
and @jerome, and it looks like that all those problems can be addressed
in one go if we run tests under user namespaces with private mounts for
/tmp and /dev/shm.

Even though namespaces is generally no-go in Nexedi, they seem to be ok
to use in tests. For example they are already used via private_tmpfs
option in SlapOS:

https://lab.nexedi.com/nexedi/slapos/blob/1876c150/slapos/recipe/librecipe/execute.py#L87-103
https://lab.nexedi.com/nexedi/slapos/blob/1876c150/software/neoppod/instance-neo-input-schema.json#L121-124
https://lab.nexedi.com/nexedi/slapos/blob/1876c150/software/neoppod/instance-neo.cfg.in#L11-16
https://lab.nexedi.com/nexedi/slapos/blob/1876c150/software/neoppod/instance-neo.cfg.in#L30-34
https://lab.nexedi.com/nexedi/slapos/blob/1876c150/software/neoppod/instance-neo.cfg.in#L170-177
...
https://lab.nexedi.com/nexedi/slapos/blob/1876c150/stack/erp5/instance-zope.cfg.in#L227-230

Thomas says that using private tmpfs for each test would be a better
solution than implementing tmpfs for whole /tmp on testnodes. He also
reports that @jp is OK to use namespaces for test as long as there is a
fallback if namespaces aren't available.

-> So let's do that: teach nxdtest to run each test case in its own
private environment with privately-mounted /tmp and /dev/shm if we can
detect that user namespaces are available. In an environment where user
namespaces are indeed available this addresses all 3 problems because
isolation and being-tmpfs are there by design, and even if some files
will leak, the kernel will free everything when test terminates and the
filesystem is automatically unmounted. We also detect such leakage and
report a warning so that such problems do not go completely unnoticed.

Implementation

We leverage unshare(1) for simplicity. I decided to preserve uid/gid
instead of becoming uid=0 (= `unshare -Umr`) for better traceability, so
that it is clear from test output under which real slapuser a test is
run(*). Not changing uid requires to activate ambient capabilities so
that mounting filesystems, including FUSE-based needed by wendelin.core,
continue to work under regular non-zero uid. Please see
https://git.kernel.org/linus/58319057b784 for details on this topic. And
please refer to added trun.py for details on how per-test namespace is setup.

Using FUSE inside user namespaces requires Linux >= 4.18 (see
https://git.kernel.org/linus/da315f6e0398 and
https://git.kernel.org/linus/8cb08329b080), so if we are really to use
this patch we'll have to upgrade kernel on our testnodes, at least where
wendelin.core is used in tests.

"no namespaces" detection is implemented via first running `unshare ...
true` with the same unshare options that are going to be used to create
and enter new user namespace for real. If that fails, we fallback into
"no namespaces" mode where no private /tmp and /dev/shm are mounted(%).

(*) for example nxdtest logs information about the system on startup:

    date:   Mon, 29 Nov 2021 17:27:04 MSK
    xnode:  slapuserX@test.node
    ...

(%) Here is how nxdtest is run in fallback mode on my Debian 11 with
    user namespaces disabled via `sysctl kernel.unprivileged_userns_clone=0`

    (neo) (z-dev) (g.env) kirr@deca:~/src/wendelin/nxdtest$ nxdtest
    date:   Thu, 02 Dec 2021 14:04:30 MSK
    xnode:  kirr@deca.navytux.spb.ru
    uname:  Linux deca 5.10.0-9-amd64 #1 SMP Debian 5.10.70-1 (2021-09-30) x86_64
    cpu:    Intel(R) Core(TM) i7-7600U CPU @ 2.80GHz

    >>> pytest
    $ python -m pytest
    # user namespaces not available. isolation and many checks will be deactivated.    <--- NOTE
    ===================== test session starts ======================
    platform linux2 -- Python 2.7.18, pytest-4.6.11, py-1.10.0, pluggy-0.13.1
    rootdir: /home/kirr/src/wendelin/nxdtest
    plugins: timeout-1.4.2
    collected 23 items

    nxdtest/nxdtest_pylint_test.py ....                      [ 17%]
    nxdtest/nxdtest_pytest_test.py ...                       [ 30%]
    nxdtest/nxdtest_test.py ......xx                         [ 65%]
    nxdtest/nxdtest_unittest_test.py ........                [100%]

    ============= 21 passed, 2 xfailed in 2.67 seconds =============
    ok      pytest  3.062s  # 23t 0e 0f 0s
    # ran 1 test case:  1·ok

/helped-by @tomo, @jerome