Pull Kselftest fixes from Mickaël Salaün:
 "Fix Kselftest's vfork() side effects.

  As reported by Kernel Test Robot and Sean Christopherson, some
  tests fail since v6.9-rc1 . This is due to the use of vfork() which
  introduced some side effects. Similarly, while making it more generic,
  a previous commit made some Landlock file system tests flaky, and
  subject to the host's file system mount configuration.

  This fixes all these side effects by replacing vfork() with clone3()
  and CLONE_VFORK, which is cleaner (no arbitrary shared memory) and
  makes the Kselftest framework more robust"

Link: https://lore.kernel.org/oe-lkp/202403291015.1fcfa957-oliver.sang@intel.com
Link: https://lore.kernel.org/r/ZjPelW6-AbtYvslu@google.com
Link: https://lore.kernel.org/r/20240511171445.904356-1-mic@digikod.net

* tag 'kselftest-fix-vfork-2024-05-12' of git://git.kernel.org/pub/scm/linux/kernel/git/mic/linux:
  selftests/harness: Handle TEST_F()'s explicit exit codes
  selftests/harness: Fix vfork() side effects
  selftests/harness: Share _metadata between forked processes
  selftests/pidfd: Fix wrong expectation
  selftests/harness: Constify fixture variants
  selftests/landlock: Do not allocate memory in fixture data
  selftests/harness: Fix interleaved scheduling leading to race conditions
  selftests/harness: Fix fixture teardown
  selftests/landlock: Fix FS tests when run on a private mount point
  selftests/pidfd: Fix config for pidfd_setns_test