[duneci-standard-test] Restructure to make default values visible.

Ansgar prefers a pattern of the form
```sh
VAR=default_value
if explicitly_configured; then
  VAR=configured_value
fi
```
This restructures the labels parsing the make this pattern possible.

base-common/duneci-standard-test

@@ -21,43 +21,48 @@ if [[ -v DUNECI_PARALLEL ]]; then
   parallel_opts="-j${DUNECI_PARALLEL}"
 fi
 
-# Evaluate $DUNECI_TEST_LABELS and set $build_test_target and
-# $labels_regex_arg accordingly.  Labels in $DUNECI_TEST_LABELS may be
-# seperated by whitespace or ','.
-handle_labels()
+# Parse labels in $1 and set ${parsed_targets[@]} and ${parsed_selectors[@]}
+# accordingly.  Labels in $1 may be seperated by whitespace or ','.
+parse_labels()
 {
   # use a function to automatically restore IFS
-  local IFS=$IFS, label labels labels_regexs
-  labels=(${DUNECI_TEST_LABELS-})
+  local IFS="$IFS," label regexs status=1
 
-  if [[ ${#labels[@]} -eq 0 ]]; then
-    build_test_targets=(build_tests)
-    labels_regex_arg=
-  else
-    build_test_targets=()
-    labels_regexs=()
-    for label in "${labels[@]}"; do
-      # ensure the labels can be safely handled
-      if [[ -z $label ]] || [[ $label == *[![:word:]-]* ]]; then
-          cat >&2 <<EOF
+  parsed_targets=()
+  regexs=()
+  for label in $1; do
+    # ensure the labels can be safely handled
+    if [[ -z $label ]] || [[ $label == *[![:word:]-]* ]]; then
+      cat >&2 <<EOF
 duneci-standard-test: Error: Invalid label in DUNECI_TEST_LABELS: '$label'
 duneci-standard-test: Note: Only alphanumeric characters plus '_' and '-' are
 duneci-standard-test: Note: allowed in label names, and the names must be
 duneci-standard-test: Note: non-empty.
 EOF
-          exit 2
-      fi
-      build_test_targets+=("build_${label}_tests")
-      labels_regexs+=("^${label}\$")
-    done
+      exit 2
+    fi
+    parsed_targets+=("build_${label}_tests")
+    regexs+=("^${label}\$")
+    status=0 # we found at least one label
+  done
+
+  parsed_selectors=()
+  if [[ ${#regexs[@]} -gt 0 ]]; then
     # This will join all regexps using '|' from IFS
     IFS="|"
-    labels_regex_arg="${labels_regexs[*]}"
-    # This will quote the result for when dunecontrol evals the parameters
-    labels_regex_arg="-L ${labels_regex_arg@Q}"
+    parsed_selectors=(-L "${regexs[*]}")
   fi
+
+  return $status
 }
-handle_labels
+
+build_test_targets=(build_tests) # passed to make/ninja to build tests
+select_test_args=()              # passed to ctest to select tests
+if parse_labels "${DUNECI_TEST_LABELS-}"; then
+    build_test_target=("${parsed_targets[@]}")
+    select_test_args=("${parsed_selectors[@]}")
+fi
+
 
 # Allow oversubscription (tests might want to try having more ranks
 # than environment has processors) and force degraded mode (as we
@@ -74,4 +79,5 @@ set -x
 ${DUNECONTROL} --current "${@}" vcsetup
 ${DUNECONTROL} --current "${@}" configure
 ${DUNECONTROL} --current "${@}" make ${parallel_opts} all "${build_test_targets[@]}"
-${DUNECONTROL} --current "${@}" bexec ${DUNE_CTEST} ${parallel_opts} "$labels_regex_arg"
+# note the extra quoting for select_test_args to protect it from dunecontrol evaling it
+${DUNECONTROL} --current "${@}" bexec ${DUNE_CTEST} ${parallel_opts} "${select_test_args[@]@Q}"