bpo-39299: Add more tests for mimetypes and its cli. (GH-17949)

* Add tests for case insensitive check of types and extensions as fallback.
* Add tests for data url with no comma.
* Add tests for read_mime_types.
* Add tests for the mimetypes cli and refactor __main__ code to private function.
* Restore mimetypes.knownfiles value at the end of the test.