[WIP] plugin fixtures: return testrunner result

[ltalirz]

This is needed in order e.g. to fail on travis, when tests don't pass.

[ltalirz]

@DropD The travis test of the PluginTestCase (using the TestRunner) did not actually fail, when the tests failed.
This has now been fixed, but it turns out that there is a problem with the PluginTestCase in sqlalchemy [1].
Could you please look into it? I have given you write access to my fork, so you can work directly in the branch.

[1] See https://travis-ci.org/ltalirz/aiida_core/jobs/395127368#L1502

[codecov-io]

Codecov Report

Merging #1676 into release_v0.12.2 will decrease coverage by 0.01%.
The diff coverage is 83.33%.

@@                 Coverage Diff                 @@
##           release_v0.12.2    #1676      +/-   ##
===================================================
- Coverage            54.66%   54.65%   -0.02%     
===================================================
  Files                  246      246              
  Lines                32420    32423       +3     
===================================================
- Hits                 17723    17721       -2     
- Misses               14697    14702       +5

Impacted Files	Coverage Δ
aiida/utils/fixtures.py	89.5% <83.33%> (-0.35%)	⬇️
aiida/backends/sqlalchemy/__init__.py	85.71% <0%> (-14.29%)	⬇️
aiida/orm/implementation/django/computer.py	72.19% <0%> (-1.47%)	⬇️

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