selftests/harness: Share _metadata between forked processes

Unconditionally share _metadata between all forked processes, which
enables to actually catch errors which were previously ignored.

This is required for a following commit replacing vfork() with clone3()
and CLONE_VFORK (i.e. not sharing the full memory) .  It should also be
useful to share _metadata to extend expectations to test process's
forks.  For instance, this change identified a wrong expectation in
pidfd_setns_test.

Because this _metadata is used by the new XFAIL_ADD(), use a global
pointer initialized in TEST_F().  This is OK because only XFAIL_ADD()
use it, and XFAIL_ADD() already depends on TEST_F().

Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Will Drewry <wad@chromium.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20240511171445.904356-9-mic@digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>