Cancel test run on SIGINT/SIGTERM

In addition to canceling test run is master tells us to do so, also cancel the
run if interrupted or terminated.

With the following sample .nxdtest

    TestCase('sleep', ['sleep', '10'])

before the patch it does not react to CTRL+C:

    $ nxdtest
    ...
    >>> sleep
    $ sleep 10
    ^C^C^C^C^C^C^C^C^C^C^C^C^C^C^C^C^C^C^C^C^C^C^C^C^C# ran 0 test cases.   <-- no reaction to CTRL+C, finishes after 10 seconds
    Traceback (most recent call last):
      File "/home/kirr/src/tools/go/py3.venv2/bin/nxdtest", line 33, in <module>
        sys.exit(load_entry_point('nxdtest', 'console_scripts', 'nxdtest')())
      File "/home/kirr/src/tools/go/py3.venv2/lib/python3.9/site-packages/decorator.py", line 232, in fun
        return caller(func, *(extras + args), **kw)
      File "/home/kirr/src/tools/go/pygolang-master/golang/__init__.py", line 103, in _
        return f(*argv, **kw)
      File "/home/kirr/src/wendelin/nxdtest/nxdtest/__init__.py", line 339, in main
        wg.wait()
    KeyboardInterrupt

after the patch:

    $ nxdtest
    ...
    >>> sleep
    $ sleep 10
    ^C# Interrupt					<-- prompt reaction to CTRL+C
    # stopping due to cancel
    # leaked pid=188877 'sleep' ['sleep', '10']
    error   sleep   1.030s  # 1t 1e 0f 0s
    # test run canceled
    # ran 1 test case:  1·error

Needs nexedi/pygolang!17 to work.

/reviewed-by @jerome
/reviewed-on nexedi/nxdtest!16