Convert test/test_util.py to pytest

[aucampia]

Fairly straight forward conversion from unittest to pytest.

This also fixes some issues that were present in the old tests:

• assertTrue was used instead of assertEqual inside:
  • test_util_from_n3_expectliteralwithdatatypefromint
  • test_util_from_n3_expectliteralwithdatatypefrombool
  • test_util_from_n3_expectliteralmultiline
  • test_util_from_n3_expectliteralwithescapedquote
• test_util_from_n3_expectliteralwithescapedquote was not comparing
  against the right literal, but this was missed because it was using
  assertTrue instead of assertEqual
• test_util_from_n3_not_escapes_xf had ("\\I") as a value but this
  was not failing, so it has been moved to test_util_from_n3_not_escapes.

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[aucampia]

I kept this very conservative, there are much more that can be done to improve all tests, but I also want to make it easy to review, so this is just doing a 1:1 conversion except for places where there are actual problems not just stylistic concerns.