Convert some tests to pytest style

[jdufresne]

To better follow conventional pytest style, this removes the outer
wrapper class in favor of a function for some tests. These tests were
picked as they are relatively simple and presented no barriers to a
quick port. The assert* methods are replaced with assert statements.
When necessary, a fixture is used to create a temporary directory.

This commit does not convert the entire test suite to this style as some
test classes use methods or other advanced features that are difficult
to automatically convert. The goal is to address these issues in
followup commits.

Refs #4193

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To reviewers, I'm happy expand or shrink the scope of this change as desired. If there is a preference for this to be split up over multiple commits, let me know. I'm happy to make adjustments.

[hugovk]

Noting something new to me, this looks handy:

The tmp_path fixture

https://docs.pytest.org/en/latest/tmpdir.html

[hugovk]

I'm fine with this. Any comments from anyone else?

---

I tried timing running the changed tests in this commit (using pytest) vs. the previous commit (using unittest), and the numbers were more or less inconclusive.

• 3m49s pytest
• 3m58s unittest
• 3m41s unittest
• 3m47s unittest
• 4m04s pytest
• 4m05s pytest
• 4m05s unittest
• 4m56s unittest

for i in {1..100}; do pytest Tests/test_000_sanity.py Tests/test_binary.py Tests/test_box_blur.py Tests/test_core_resources.py Tests/test_file_bufrstub.py Tests/test_file_container.py Tests/test_file_gd.py Tests/test_file_gimppalette.py Tests/test_file_wal.py Tests/test_format_lab.py Tests/test_image_getbands.py Tests/test_image_getbbox.py Tests/test_image_getim.py Tests/test_image_getpalette.py Tests/test_image_putpalette.py Tests/test_image_tobytes.py Tests/test_imagestat.py Tests/test_locale.py Tests/test_main.py Tests/test_util.py; done

[hugovk]

Alright, let's go ahead with this. Thanks!