[tests] Update l10n_tests.py
[tests] Update l10n_tests.py

- discard site tests because it does not care whether a site exists with
  the given language code. Probably the language is used by incubator
- test the package anyway; add two keys for the tests.

Bug: T298388
Change-Id: Ib80b7b5a488498e9de3c9dcfbf51a8b3ab8db3ed

wikimedia/pywikibot-core,wikimedia/pywikibot-core