Add a kunit_fail_current_test() function to fail the currently running
test, if any, with an error message.

This is largely intended for dynamic analysis tools like UBSAN and for
fakes.
E.g. say I had a fake ops struct for testing and I wanted my `free`
function to complain if it was called with an invalid argument, or
caught a double-free. Most return void and have no normal means of
signalling failure (e.g. super_operations, iommu_ops, etc.).

Key points:
* Always update current->kunit_test so anyone can use it.
  * commit 83c4e7a0 ("KUnit: KASAN Integration") only updated it for
  CONFIG_KASAN=y

* Create a new header <kunit/test-bug.h> so non-test code doesn't have
to include all of <kunit/test.h> (e.g. lib/ubsan.c)

* Forward the file and line number to make it easier to track down
failures

* Declare the helper function for nice __printf() warnings about mismatched
format strings even when KUnit is not enabled.

Example output from kunit_fail_current_test("message"):
[15:19:34] [FAILED] example_simple_test
[15:19:34]     # example_simple_test: initializing
[15:19:34]     # example_simple_test: lib/kunit/kunit-example-test.c:24: message
[15:19:34]     not ok 1 - example_simple_test

Fixed minor check patch with checkpatch --fix option:
Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Daniel Latypov <dlatypov@google.com>
Signed-off-by: Uriel Guajardo <urielguajardo@google.com>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>