kunit: Add test for static stub

kunit: Add test for static stub

__kunit_activate_static_stub() works effectively as
kunit_deactivate_static_stub() if `replacement_addr` is NULL.

Add a test case to catch the issue discovered in
commit 772e50a76ee6 ("kunit: Fix wrong parameter to kunit_deactivate_static_stub()").

For running the test:

$ ./tools/testing/kunit/kunit.py run \
        --arch=x86_64 \
        kunit_stub

Fixed change log:
Shuah Khan <skhan@linuxfoundation.org>

Link: https://lore.kernel.org/r/20250612084834.587576-1-tzungbi@kernel.org
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>