kunit: add tests for filtering attributes

Add four tests to executor_test.c to test behavior of filtering attributes.

- parse_filter_attr_test - to test the parsing of inputted filters

- filter_attr_test - to test the filtering procedure on attributes

- filter_attr_empty_test - to test the behavior when all tests are filtered
  out

- filter_attr_skip_test - to test the configurable filter_action=skip
  option

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