Add test for a WordLevel tokenizer model
# What does this PR do?

In this PR I propose to add a test for the feature developed in PR #12361. Unless I'm mistaken, no language model tested currently uses a tokenizer that would use the WordLevel model. 

The tokenizer created for this test is hosted here: [https://hf.co/robot-test/dummy-tokenizer-wordlevel](https://huggingface.co/robot-test/dummy-tokenizer-wordlevel)
