Replace all 45 sct_testing tests with Pytest equivalents

[joshuacwnewton]

Checklist

GitHub

• I've given this PR a concise, self-descriptive, and meaningful title
• I've linked relevant issues in the PR body
• I've applied the relevant labels to this PR
• I've applied a release milestone (major, minor, patch) in line with Semantic Versioning guidelines
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PR contents

• I've consulted SCT's internal developer documentation to ensure my contribution is in line with any relevant design decisions
• I've added relevant tests for my contribution
• I've updated the relevant documentation for my changes, including argparse descriptions, docstrings, and ReadTheDocs tutorial pages

Description

This PR replaces sct_testing with pytest. The motivation for this is summarized here: #3093 (comment)

45 of the commits in this PR do the following:

• Removes an original sct_testing test.
• Removes a "backwards compatibility" wrapper test.
• Adds a new pytest test to replace these older tests.

Then, there are 4 additional commits:

• f0defbf: Adds a fixture to ensure that the tests run in the correct directory.
• 762af7f: Adds a custom marker so that pytest -m sct_testing behaves identically to sct_testing.
• b78ce08: Fixes a bug in sct_register_multimodal.py discovered by the new tests.
• d06f961: Removes spinalcordtoolbox/scripts/sct_testing.py itself.

How to review this PR

This PR is quite large, and I apologize for that. But, I think it's best to do this in one go. If we try to split up the PR, we will have a half-working sct_testing if only one PR is merged. (If you have any ideas for how to make this easier to review, please let me know. 😅)

To review this PR, please compare each old sct_testing test to the new pytest test to make sure they match up. (For example, compare testing/test_sct_analyze_lesion.py with unit_testing/cli/test_cli_sct_analyze_lesion.py.)

To make this easier, use "Commit" view, because each commit will contain a pair of test files.

Note: The old sct_testing tests can be a bit confusing. So, to help explain, I have written a Wiki page called sct_testing.

Linked issues

Fixes #3093.
Fixes #2133.

[joshuacwnewton]

For transparency, I'm going to rebase the changes from 0e3434e, 4c8d34d, f6ab574, and ee44471 to make sure that the commit view is still useful for comparing the tests from this PR.

[joshuacwnewton]

Thank you so, so much for taking the time to provide such detailed feedback, @kousu.

I think I've answered all of your feedback. If it's not too much trouble, could you go through your comments and mark them as "resolved" if you're satisfied by the changes? 🙂

If everything looks good, I can rebase the bugfixes into the main commits so that the "Commit" view is useful again. (Or, maybe I can just merge at this point -- there's diminishing returns to multiple rounds of reviews here, especially because we're going to continue to update these tests with more thorough checks.)