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-build-tests

@@ -26,4 +26,3 @@ spack config get compilers
 
 # Run some build smoke tests, potentially with code coverage
 ${coverage_run} bin/spack install ${SPEC}
-${coverage_combine}