coverage: only run coverage for subset of unit tests

- Codecov cannot handle as many coverage reports as we are generating

- as a result, our PR coverage pages have been broken for a while, and
  it's hard to tell people where to enhance their testing in PR reviews.

- Scale back to only running coverage for 3.7 and 2.7 unit tests

- This is *probably* better.  We run the build tests for good measure,
  but we do not need to evaluate them for coverage.  The coverage reports
  are about unit tests.

share/spack/qa/setup.sh

@@ -16,8 +16,12 @@ export SPACK_ROOT=$(realpath "$QA_DIR/../../..")
 # Source the setup script
 . "$SPACK_ROOT/share/spack/setup-env.sh"
 
+# by default coverage is off.
+coverage=""
+coverage_run=""
+
 # Set up some variables for running coverage tests.
-if [[ "$TEST_SUITE" == "unit" || "$TEST_SUITE" == "build" ]]; then
+if [[ "$COVERAGE" == "true" ]]; then
     # these set up coverage for Python
     coverage=coverage
     coverage_run="coverage run"
@@ -33,9 +37,6 @@ if [[ "$TEST_SUITE" == "unit" || "$TEST_SUITE" == "build" ]]; then
         bashcov=$(realpath ${QA_DIR}/bashcov)
         sed -i~ "s@#\!/bin/bash@#\!${bashcov}@" "$cc_script"
     fi
-else
-    coverage=""
-    coverage_run=""
 fi
 
 #