selftests/harness: Fix tests timeout and race condition

We cannot use CLONE_VFORK because we also need to wait for the timeout
signal.

Restore tests timeout by using the original fork() call in __run_test()
but also in __TEST_F_IMPL().  Also fix a race condition when waiting for
the test child process.

Because test metadata are shared between test processes, only the
parent process must set the test PID (child).  Otherwise, t->pid may be
set to zero, leading to inconsistent error cases:

  #  RUN           layout1.rule_on_mountpoint ...
  # rule_on_mountpoint: Test ended in some other way [127]
  #            OK  layout1.rule_on_mountpoint
  ok 20 layout1.rule_on_mountpoint

As safeguards, initialize the "status" variable with a valid exit code,
and handle unknown test exits as errors.

The use of fork() introduces a new race condition in landlock/fs_test.c
which seems to be specific to hostfs bind mounts, but I haven't found
the root cause and it's difficult to trigger.  I'll try to fix it with
another patch.

Cc: Christian Brauner <brauner@kernel.org>
Cc: Günther Noack <gnoack@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Will Drewry <wad@chromium.org>
Cc: stable@vger.kernel.org
Closes: https://lore.kernel.org/r/9341d4db-5e21-418c-bf9e-9ae2da7877e1@sirena.org.uk
Fixes: a86f1890 ("selftests/harness: Fix interleaved scheduling leading to race conditions")
Fixes: 24cf65a6

 ("selftests/harness: Share _metadata between forked processes")
Link: https://lore.kernel.org/r/20240621180605.834676-1-mic@digikod.net
Tested-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Mickaël Salaün <mic@digikod.net>