Merge patch series "selftests: openat2: migrate to kselftest harness"

Aleksa Sarai <aleksa@amutable.com> says:

These tests were written in the early days of selftests' TAP support,
the more modern kselftest harness is much easier to follow and maintain.
The actual contents of the tests are unchanged by this change.

* patches from https://patch.msgid.link/20260401-openat2-selftests-kunit-v2-0-ad153a07da0c@amutable.com:
  selftests: openat2: migrate to kselftest harness
  selftests: openat2: switch from custom ARRAY_LEN to ARRAY_SIZE
  selftests: openat2: move helpers to header
  selftests: move openat2 tests to selftests/filesystems/

Link: https://patch.msgid.link/20260401-openat2-selftests-kunit-v2-0-ad153a07da0c@amutable.com
Signed-off-by: Christian Brauner <brauner@kernel.org>