Restore multiprocessing in unit tests (#6949)

* Revert "Travis: use --concurrency=multiprocessing only on build tests (#6872)"

This reverts commit 596d4637.

* Removing 'coverage combine' in test script

According to what was discovered in #6887, one of the problems is
calling 'coverage combine' twice without the '-a' flag. This removes
the first call within our test scripts.