Add CI check for no partial karma jest migration

scripts/frontend/check_no_partial_karma_jest.sh

+#!/usr/bin/env bash
+
+karma_files=$(find spec/javascripts ee/spec/javascripts -type f -name '*_spec.js' -not -path '*/helpers/*')
+violations=""
+
+for karma_file in $karma_files; do
+  jest_file=${karma_file/spec\/javascripts/"spec/frontend"}
+
+  if [ -f $jest_file ]; then
+    violations="$violations $jest_file"
+  fi
+done
+
+if [[ -z "$violations" ]]; then
+  echo "All good!"
+  exit 0
+else
+  echo "Danger! The following Jest specs have corresponding files in the Karma spec directory (i.e. spec/javascripts):"
+  echo ""
+  echo "------------------------------"
+  for file in $violations; do
+    echo $file
+  done
+  echo "------------------------------"
+  echo ""
+  echo "For each of these files, please either:"
+  echo ""
+  echo "1. Fully migrate the file to Jest and remove the corresponding Karma file."
+  echo "2. Remove the Jest file for now, make any relevant changes in the corresponding Karma file, and handle the migration to Jest in a separate MR."
+  echo ""
+  echo "Why is this a problem?"
+  echo ""
+  echo "- It's nice to have a single source of truth for the unit tests of a subject."
+  echo "- This will cause conflicts if the remaining Karma spec is migrated using our automated tool."
+  echo "  https://gitlab.com/gitlab-org/frontend/playground/migrate-karma-to-jest"
+  echo ""
+  exit 1
+fi

scripts/static-analysis

@@ -35,7 +35,8 @@ def jobs_to_run(node_index, node_total)
     %w[yarn run prettier-all],
     %w[bundle exec rubocop --parallel],
     %w[scripts/lint-conflicts.sh],
-    %w[scripts/lint-rugged]
+    %w[scripts/lint-rugged],
+    %w[scripts/frontend/check_no_partial_karma_jest.sh]
   ]
 
   case node_total