Restore `test_apply_everywhere_floordivide`

[dhomeier]

Started this as experimental follow-up to #783 to restore the floordivide test, which seems to have failed due to unrelated open file exceptions. Additional open-file failures reported in https://github.com/radio-astro-tools/spectral-cube/pull/783#pullrequestreview-832819170 are fixed in a5f218f, although I could reproduce these only in local to runs under macOS. Both 6c9a4dc and a5f218f are passing all tests except the unrelated doctest failure; moving the pytest.skip() before any file read operations are started in the 2nd commit seems less error-prone in general, but since this did not appear to cause problems in CI, feel free to pick only the 1st one.

[codecov-commenter]

Codecov Report

Merging #788 (a5f218f) into master (7452dce) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #788   +/-   ##
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  Coverage   77.90%   77.90%           
=======================================
  Files          24       24           
  Lines        5826     5826           
=======================================
  Hits         4539     4539           
  Misses       1287     1287           

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[dhomeier]

One complication was probably that open_files seems to have everything run in a sort of sandbox, making it almost impossible to track the origins of the failures to the respective tests. The twod_numpy tests, as far as I could see, were starting e.g. with test_twod_numpy[True-sum-slice-0-data_advs], which is skipped, but with the file already opened, and that probably interfered with the open-files check for the next test_twod_numpy[True-sum-cube-0-data_advs] in my local installation.