Update seviri icare tests

[simonrp84]

The seviri icare reader had poor test coverage, and quite a few things weren't being tested at all.

This PR updates the tests to fix the above, and now tests everything of importance.

• Tests added
• Passes flake8 satpy
• Fully documented

[coveralls]

Coverage increased (+0.2%) to 90.393% when pulling 30af77a on simonrp84:icare_tests into e77cd29 on pytroll:master.

[codecov]

Codecov Report

Merging #1342 into master will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1342      +/-   ##
==========================================
+ Coverage   90.24%   90.39%   +0.15%     
==========================================
  Files         222      228       +6     
  Lines       32719    33017     +298     
==========================================
+ Hits        29526    29845     +319     
+ Misses       3193     3172      -21     

Impacted Files	Coverage Δ
satpy/tests/reader_tests/test_seviri_l1b_icare.py	100.00% <100.00%> (+1.04%)	⬆️
satpy/tests/reader_tests/test_netcdf_utils.py	94.79% <0.00%> (-0.82%)	⬇️
satpy/composites/__init__.py	82.76% <0.00%> (-0.17%)	⬇️
satpy/readers/yaml_reader.py	95.07% <0.00%> (-0.17%)	⬇️
satpy/scene.py	90.42% <0.00%> (-0.16%)	⬇️
satpy/dependency_tree.py	97.26% <0.00%> (-0.01%)	⬇️
satpy/tests/test_yaml_reader.py	99.83% <0.00%> (-0.01%)	⬇️
satpy/tests/utils.py	95.40% <0.00%> (ø)
satpy/writers/cf_writer.py	93.77% <0.00%> (ø)
satpy/tests/test_dataset.py	100.00% <0.00%> (ø)
... and 17 more

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[mraspaud]

Good job on adding the area and other attribute checks. Just some readability issues I'd like to clarify.