kunit: Add tests for kprobe stub

For running the test:

./tools/testing/kunit/kunit.py run \
        --arch=x86_64 \
        --kconfig_add CONFIG_KPROBES=y \
        --kconfig_add CONFIG_KUNIT_KPROBE_STUBS=y \
        kunit_stub

Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>