Add unit tests to Celery tasks

[adelavega]

Fixes #615

[codecov-io]

Codecov Report

Merging #617 into master will increase coverage by 11.54%.
The diff coverage is 69.09%.

@@             Coverage Diff             @@
##           master     #617       +/-   ##
===========================================
+ Coverage   70.79%   82.33%   +11.54%     
===========================================
  Files          56       56               
  Lines        2164     2174       +10     
===========================================
+ Hits         1532     1790      +258     
+ Misses        632      384      -248

Impacted Files	Coverage Δ
neuroscout/resources/__init__.py	100% <ø> (ø)	⬆️
neuroscout/tasks/compile.py	88.5% <ø> (ø)
neuroscout/tasks/viz.py	92.3% <ø> (ø)
celery_worker/app.py	0% <0%> (ø)	⬆️
neuroscout/config/example_app.py	0% <0%> (ø)	⬆️
neuroscout/tasks/upload.py	54.41% <100%> (ø)
neuroscout/resources/analysis/reports.py	63.41% <100%> (+10.32%)	⬆️
neuroscout/worker.py	100% <100%> (ø)	⬆️
neuroscout/tasks/utils.py	97.22% <100%> (ø)
neuroscout/tasks/report.py	72.6% <100%> (ø)
... and 10 more

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