[PoC] Run the unit tests in parallel

[lslezak]

Problem

Running the unit tests takes quite long time, we should run the tests in parallel to speed it up. We already use that solution in the yast2-storage-ng package.

Todo

• Fix the missing require
• Fix the Storage::LockException problem (according to @imobachgs we should run the storage related tests in one process, the other option would be to mock the storage calls)
• Adapt the .spec file (like here)
• Fix code coverage reporting

Note: the Storage::LockException happens only with more parallel processes (16), Travis uses just 2 CPUs so it does not happen there, the affected tests coincidentally run in the same process.

Results

• On my machine the test duration went down from 30s to 10s (3x faster) (using a 8C/16T CPU)
• At Travis the speed up is smaller (only 2 CPUs) and it highly depends how fast the worker machine is (it might actaully take the same time, see parallel tests and the standard tests).
• The question is how faster it will be in OBS, but the builds usually use 4 or 8 CPUs so the speed up should be pretty good there.

[coveralls]

Coverage decreased (-23.5%) to 34.991% when pulling 092b849 on parallel_tests into 52039cb on master.