kasan: test: make use of kunit_skip()

Make use of the recently added kunit_skip() to skip tests, as it permits
TAP parsers to recognize if a test was deliberately skipped.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>