Restore multiprocessing in unit tests (#6949)

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

This reverts commit 596d463714.

* 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.

share/spack/qa/run-unit-tests

@@ -29,4 +29,3 @@ ${coverage_run} bin/spack -p --lines 20 spec mpileaks
 
 # Run unit tests with code coverage
 ${coverage_run} bin/spack test "$@"
-${coverage_combine}