[chore] Add unittests for InformationRetrievalEvaluator

[fpgmaas]

This PR does two things:

• Add unit tests for InformationRetrievalEvaluator, raising its test coverage from 11% to 86%, and raising total coverage for the project from 50.7% to 53.3%.
• Move the unit tests from test_evaluator.py to individual files in tests/evaluation, to mimic the directory structure in sentence_transformers and to easily allow running all tests for evaluators with pytest -s tests/evaluation.

Before

File	Statements	Missing	Excluded	Coverage
sentence_transformers/evaluation/InformationRetrievalEvaluator.py	211	188	0	11%

After

File	Statements	Missing	Excluded	Coverage
sentence_transformers/evaluation/InformationRetrievalEvaluator.py	211	30	0	86%

[tomaarsen]

Hello!

Apologies for the delay, I've been recovering from a surgery this last month.
Thanks a bunch for creating this test & splitting up the existing ones. I like the format a lot. This looks ready to go as far as I can tell :)

• Tom Aarsen