Fix AVHRR tests importing external mock on Python 3

[gerritholl]

In the AVHRR L1B GACLAC testing routine, do not import mock from
external package when it can be successfully imported from the unittest
module.

• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff

[coveralls]

Coverage decreased (-0.03%) to 81.813% when pulling 450ec28 on gerritholl:fix-import-mock into 7fe50a7 on pytroll:master.

[coveralls]

Coverage decreased (-0.01%) to 81.831% when pulling 450ec28 on gerritholl:fix-import-mock into 7fe50a7 on pytroll:master.

[codecov]

Codecov Report

Merging #763 into master will decrease coverage by 0.01%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #763      +/-   ##
==========================================
- Coverage   81.84%   81.83%   -0.02%     
==========================================
  Files         158      158              
  Lines       22728    22731       +3     
==========================================
  Hits        18601    18601              
- Misses       4127     4130       +3

Impacted Files	Coverage Δ
satpy/tests/reader_tests/test_avhrr_l1b_gaclac.py	94.87% <50%> (-2.47%)	⬇️
satpy/scene.py	89.91% <0%> (-0.18%)	⬇️

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