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 <[email protected]>
Cc: Günther Noack <[email protected]>
Cc: Jakub Kicinski <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Will Drewry <[email protected]>
Cc: [email protected]
Closes: https://lore.kernel.org/r/[email protected]
Fixes: a86f18903db9 ("selftests/harness: Fix interleaved scheduling leading to race conditions")
Fixes: 24cf65a62266 ("selftests/harness: Share _metadata between forked processes")
Link: https://lore.kernel.org/r/[email protected]
Tested-by: Mark Brown <[email protected]>
Signed-off-by: Mickaël Salaün <[email protected]>

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