externalTests: Replace optimizer levels with named presets

scripts/common.sh

@@ -211,6 +211,7 @@ function safe_kill
 
 function circleci_select_steps
 {
+    # We expect multiple lines in $all_steps, one step per line
     local all_steps="$1"
     (( $# == 1 )) || assertFail
 
@@ -222,6 +223,12 @@ function circleci_select_steps
     fi
 }
 
+function circleci_select_steps_multiarg
+{
+    # We expect multiple arguments, one step per argument.
+    circleci_select_steps "$(printf '%s\n' "$@")"
+}
+
 function circleci_step_selected
 {
     local selected_steps="$1"