Refactor the convoluted test scripts

[erogluorhan]

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Our test directory and scripts were initially intended to have a test script in correspondence with each code scripts we have. For some while, we seem to have lost track of this (See the below example). Also, the test directory has all of the test scripts as a plain single list, while our code dir has modules.

We can benefit from a test directory refactor to have things more readable and consistent.

For reference, see:

Regarding the existence & name of this test script: We had a tradition of creating test scripts in correspondence with the code scripts we have; this one is a bit off with that (I cannot remember if there are other test scripts in a similar case honestly). Should we be strict and simply put this test case into test_grid.py rather than creating its own file? Unsure what way personally.

Originally posted by @erogluorhan in https://github.com/UXARRAY/uxarray/pull/922#discussion_r1775973579