revocable: Add Kunit test cases

revocable: Add Kunit test cases

Add Kunit test cases for the revocable API.

The test cases cover the following scenarios:
- Basic: Verifies that a consumer can successfully access the resource
  provided via the provider.
- Revocation: Verifies that after the provider revokes the resource,
  the consumer correctly receives a NULL pointer on a subsequent access.
- Try Access Macro: Same as "Revocation" but uses the
  REVOCABLE_TRY_ACCESS_WITH() and REVOCABLE_TRY_ACCESS_SCOPED().

A way to run the test:
$ ./tools/testing/kunit/kunit.py run \
        --kconfig_add CONFIG_REVOCABLE_KUNIT_TEST=y \
        revocable_test

Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://patch.msgid.link/20260116080235.350305-3-tzungbi@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>