The plugin is often necessary, it's documented https://drupal.org/node/2135323 but not yet tested. I voluntereed mpgeek to write the test for the plugin attached.
The test should run with a string containing spaces, commas, the like, above 128 Unicode characters. It needs to create a mock transliteration object (Drupal\Component\Transliteration\TransliterationInterface and set the expectation it is called. Also check that the spaces, commas and the like are all filtered out and the a series of underscores are collapsed into one.