Adding test to check that languages.yml includes all extensions represented in samples folder

This is really just a housekeeping thing but currently the list of extensions (i.e. `language.all_extensions`) is defined by both the extensions in `languages.yml` but also the sample files for that language.

This PR adds a test to make sure that `languages.yml` extensions include all file extensions present in the samples folder for a language.

Note there should not be any behaviour change with this.