Fix MultiScene.save_animation to work with new dask.distributed versions

[djhoese]

New-ish versions of dask do not allow the distributed Client to be used the way I had originally programmed save_animation to use it. This PR fixes it to work with newer versions of dask and also updates the tests to use the real distributed Client to avoid this problem in the future.

• Tests added and test suite added to parent suite
• Tests passed
• Passes flake8 satpy

[coveralls]

Coverage decreased (-0.004%) to 85.407% when pulling 8a4e63a on djhoese:bugfix-save-animation-distributed into a6ce4df on pytroll:master.

[coveralls]

Coverage decreased (-0.004%) to 85.407% when pulling 8a4e63a on djhoese:bugfix-save-animation-distributed into a6ce4df on pytroll:master.

[codecov]

Codecov Report

Merging #938 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #938      +/-   ##
=========================================
- Coverage   85.41%   85.4%   -0.01%     
=========================================
  Files         172     172              
  Lines       26046   26046              
=========================================
- Hits        22246   22245       -1     
- Misses       3800    3801       +1

Impacted Files	Coverage Δ
satpy/tests/test_multiscene.py	97.84% <100%> (ø)	⬆️
satpy/multiscene.py	89.13% <100%> (ø)	⬆️
satpy/scene.py	90.35% <0%> (-0.18%)	⬇️

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