Improve testing coverage

[anissa111]

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Also mentioned by @anissa111 in https://github.com/UXARRAY/uxarray/discussions/88#discussioncomment-3335717

At time of writing, testing coverage according to Codecov is 64%. Additionally, some of the tests we're currently running use "magic number" correct answers. While this is fine to some extent, it would also be nice to have some easily hand-verifiable answers for:

• easier testing over a variety of edge cases
• testing on multiple, small, minimal grids for speed
• an easier time debugging
• easier development process where intermediate results are more easily verifiable
• documentation and examples in the future
• lower barrier to entry for contributors in the future

Some specific areas of proposed improvement:

• creation of "test data" that can be imported with uxarray.test_data(...) or similar
• increase tests for helpers.py, which currently has testing coverage of 27%
• more tests for edge cases rather than primarily tests that verify ideal code usage
• possible expansion of __from_verts__ to include entire, proper, grid initialization
• ideally, implementation of the verify function