Disallow tests from downloading files while running tests

[djhoese]

See #1587. Basically, developers should be writing tests that mock the necessary functions so actual data downloads don't happen while tests are being run. This PR adds a check to see if we are running tests and if the file being downloaded is a README file. The auxiliary download tests themselves use a README file as the only file to actually test downloading. If we're running tests and a download is requested then a RuntimeError is raised. Offline downloads are still allowed but must also be explicitly requested (with the configuration value).

• Closes Don't allow auxiliary downloads during tests #1587
• Tests added
• Fully documented

CC @joleenf who had fears about this in her own PR.

[codecov]

Codecov Report

Merging #1591 (4bdd41d) into master (f56a089) will decrease coverage by 0.00%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master    #1591      +/-   ##
==========================================
- Coverage   92.72%   92.72%   -0.01%     
==========================================
  Files         253      254       +1     
  Lines       37576    37600      +24     
==========================================
+ Hits        34844    34866      +22     
- Misses       2732     2734       +2     

Flag	Coverage Δ
behaviourtests	4.43% <8.33%> (+<0.01%)	⬆️
unittests	92.86% <91.66%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
satpy/conftest.py	71.42% <71.42%> (ø)
satpy/aux_download.py	88.03% <100.00%> (+0.53%)	⬆️
satpy/tests/test_data_download.py	100.00% <100.00%> (ø)

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[mraspaud]

Could we use the satpy configuration system to do this instead?

[djhoese]

In what way? A new config option or reuse the download_aux one for offline downloads? In the first case, the end result would be the same code but instead of a global variable being set by the test config module it would be setting a satpy config value.

I wanted to make sure there was a specific exception (not a warning) so that anyone writing unit tests knows exactly what is going wrong here. So that's why I didn't reuse the download_aux option.

There could be a new option that defaults to not letting tests download anything, but is toggled for the aux_download tests. This would be more flexible for future tests that would need to download something, but I'm not sure we need it right now.

[mraspaud]

LGTM