Add Missing tokenization test [electra]
# What does this PR do?

Added tokenization test for electra
_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks a lot for your PR! @ArthurZucker could you review?<|||||>@sgugger Any updates, is there something wrong?<|||||>@sgugger Done!!<|||||>Thanks for contributing 🔥 
