Starting the armi.testing modules

[john-science]

What is the change?

Here I am starting the armi.testing module.

Basically, I just move loadTestReactor() from armi.reactor.test.test_reactor to armi.testing.

Why is the change being made?

Right now, every single ARMI unit test is importable as part of the API. But that can't last forever. ARMI 1.0.0 will have a more usual API, without less importable noise.

A better paradigm to follow would be numpy. The NumPy team does not let you import their unit tests, that's madness.

Instead, they provide import numpy.testing, which houses a small set of high-quality testing utilities for people to use when testing NumPy-based code. So, we will follow NumPy's example.

progress on #1867

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Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify any new/changed code.

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes have been updated if necessary.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in pyproject.toml.

[john-science]

Again, @opotowsky, I'm sorry this touches so many files. BUT, the only two non-trivial files changes are armi/testing/__init__.py and armi/reactor/tests/test_reactor.py. I move the loadTestReactor() function from one to the other.

The rest is just me making use of the new location.

But the change is still backward compatible for projects outside ARMI, because test_reactors.py still imports loadTestReactor().

Sorry for the PR onslaught!

[opotowsky]

I merged the ruff PR so you have a number of merge conflicts here. But feel free to merge this once those are handled!