kunit: add tests for using current KUnit test field

Create test suite called "kunit_current" to add test coverage for the use
of current->kunit_test, which returns the current KUnit test.

Add two test cases:
- kunit_current_test to test current->kunit_test and the method
  kunit_get_current_test(), which utilizes current->kunit_test.

- kunit_current_fail_test to test the method
  kunit_fail_current_test(), which utilizes current->kunit_test.

Signed-off-by: Rae Moar <rmoar@google.com>
Reviewed-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>