[pytx][mlh] Convert some tests in threatexchange/signal_type/ to py.test

[Dcallies]

https://github.com/facebook/ThreatExchange/blob/main/python-threatexchange/threatexchange/signal_type/tests/test_hash_from_x.py
https://github.com/facebook/ThreatExchange/blob/main/python-threatexchange/threatexchange/signal_type/tests/test_md5_hash.py
https://github.com/facebook/ThreatExchange/blob/main/python-threatexchange/threatexchange/signal_type/tests/test_pdq_index.py
https://github.com/facebook/ThreatExchange/blob/main/python-threatexchange/threatexchange/signal_type/tests/test_raw_text.py
https://github.com/facebook/ThreatExchange/blob/main/python-threatexchange/threatexchange/signal_type/tests/test_url_md5_hash.py

These tests are using unittest, we want to use py.test.

py.test docs: https://docs.pytest.org/en/stable/

[ZeyadTarekk]

I will work on that