Fix LUKE tests
# What does this PR do?

3 tests defined in `test_tokenization_luke.py` were having a timeout because they were too slow:

```
FAILED tests/test_tokenization_luke.py::Luke::test_add_special_tokens
FAILED tests/test_tokenization_luke.py::Luke::test_maximum_encoding_length_pair_input
FAILED tests/test_tokenization_luke.py::Luke::test_maximum_encoding_length_single_input
```

This was caused by the `get_clean_sequence` method (used in each of those methods), which is defined in `test_tokenization_common.py` and was inherited by default. By overwriting this method with a much simpler one, the tests are significantly faster. No bottleneck anymore.