Fix tests where xarray was unable to guess backend engine

[djhoese]

Our unstable CI tests started failing because pydata/xarray#4989 was merged. Xarray was no longer able to guess the backend engine for our tests that weren't using the .nc suffix even though the tests were created/using netcdf files. This PR fixes these tests.

• Tests added
• Fully documented

[codecov]

Codecov Report

Merging #1592 (ece20d4) into master (f56a089) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1592   +/-   ##
=======================================
  Coverage   92.72%   92.72%           
=======================================
  Files         253      253           
  Lines       37576    37577    +1     
=======================================
+ Hits        34844    34845    +1     
  Misses       2732     2732           

Flag	Coverage Δ
behaviourtests	4.42% <0.00%> (-0.01%)	⬇️
unittests	92.86% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
satpy/tests/reader_tests/test_vii_base_nc.py	98.73% <100.00%> (ø)
satpy/tests/reader_tests/test_vii_l1b_nc.py	100.00% <100.00%> (ø)
satpy/tests/reader_tests/test_vii_l2_nc.py	100.00% <100.00%> (ø)
satpy/tests/writer_tests/test_cf.py	99.69% <100.00%> (+<0.01%)	⬆️

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[mraspaud]

LGTM thanks for the quick reaction!