Remove use of SentencePieceTrainer from modeling tests

[mattdangerw]

SentencePieceTrainer is cumbersome to call, and not available in all the environments we would like to run our testing.

We should remove the usage, either by

• Hard coding "text proto" sentencepiece proto objects in our modeling test files (can you do this in python?)
• Or saving the small proto files we need for testing in tests/test_data and passing the path directly when running modeling tests.