Migrate FCI tests to pytest

[gerritholl]

Migrate FCI tests to pytest. This enables the more flexible set of
fixtures available there. This will be needed by at least one of the
tests to be added in #1177

• Closes #xxxx
• Tests added
• Tests passed
• Passes flake8 satpy
• Fully documented

[gerritholl]

CodeFactor is wrong to complain here. PEP 8 prohibits a bare test of if len(x). It does not prohibit an explicit length test such as if len(x) > 0:

# Correct:
if not seq:
if seq:

# Wrong:
if len(seq):
if not len(seq):

Since pylint-dev/pylint#2815 (apparently not merged in the version CodeFactor uses), pylint reportedly no longer complains about explicit comparisons of len(x) > 0. Therefore, I propose this particular recommendation should be ignored.

[mraspaud]

LGTM, just one typo.

[gerritholl]

The test is failing because the previous test contained a bug, which I serendipitously fixed:

satpy/satpy/tests/reader_tests/test_fci_l1c_fdhsi.py

Line 261 in a82e4a6

self.assertTrue(4, len(files))

This should have been assertEqual, and it would have failed, because we have len(files) == 5. In the new version it is

https://github.com/gerritholl/satpy/blob/7a8a121bce2a81e6af68652239f293edafde96cd/satpy/tests/reader_tests/test_fci_l1c_fdhsi.py#L256

which is correct.

[coveralls]

Coverage increased (+0.0004%) to 89.618% when pulling b51350e on gerritholl:fci-goes-pytest into 79ea923 on pytroll:master.

[mraspaud]

LGTM

[codecov]

Codecov Report

Merging #1180 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1180   +/-   ##
=======================================
  Coverage   89.61%   89.61%           
=======================================
  Files         200      200           
  Lines       29522    29523    +1     
=======================================
+ Hits        26457    26458    +1     
  Misses       3065     3065           

Impacted Files	Coverage Δ
satpy/tests/reader_tests/test_fci_l1c_fdhsi.py	100.00% <100.00%> (ø)

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