993 progress: convert 3 more files to pytest (#1520)

* 993 convert test_item_assets.py to pytest

* 993 progress on test_item_collection.py:

* replace unittest asserts with pytest asserts
* remove maxDiff

* 993 remove class (nonfunctional atm)

* 993 remove self from methods (still not functional)

* 993 install fixtures & finish first test conversion

* 993 complete conversion to pytest for test_item_collection.py

* 993 pre-commit fixups

* 993 start on test_collection.py

* 993 revise assertion

* 993 replace assertEqual with assert; cmd line:

sed -i.bak -E 's/self.assertEqual\((.*), (.*)\)/assert \1 == \2/' tests/test_collection.py

note needed a bit of editing as multiple commas in one line could confuse sed

* 993 convert rest of asserts

* 993 convert CollectionTest to top-level functions

* 993 remove self to prep for declassification

* 993 convert ExtentTest method tests to functions

* 993 convert Collection subclass tests to pytest

* 993 complete test_collection.py conversion to pytest

* 993 fastidious revisions by ruff