selftests: openat2: move helpers to header

selftests: openat2: move helpers to header

This is a bit ugly, but in the next patch we will move to using
kselftest_harness.h -- which doesn't play well with being included in
multiple compilation units due to duplicate function definitions.

Not including kselftest_harness.h would let us avoid this patch, but the
helpers will need include kselftest_harness.h in order to switch to
TH_LOG.

Signed-off-by: Aleksa Sarai <aleksa@amutable.com>
Link: https://patch.msgid.link/20260401-openat2-selftests-kunit-v2-2-ad153a07da0c@amutable.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>