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}

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}

share/spack/qa/setup.sh

@@ -11,18 +11,12 @@ SPACK_ROOT="$QA_DIR/../../.."
 . "$SPACK_ROOT/share/spack/setup-env.sh"
 
 # Set up some variables for running coverage tests.
-if [[ "$COVERAGE" == "true" && "$TEST_SUITE" == "unit" ]]; then
+if [[ "$COVERAGE" == true ]]; then
     coverage=coverage
     coverage_run="coverage run"
-    coverage_combine="coverage combine"
-elif [[ "$COVERAGE" == "true" && "$TEST_SUITE" == "build" ]]; then
-    coverage=coverage
-    coverage_run="coverage run --concurrency=multiprocessing"
-    coverage_combine="coverage combine"
 else
     coverage=""
     coverage_run=""
-    coverage_combine=""
 fi
 
 #