Look into slow Jest tests

We have a few Jest tests where the default timeout wasn't enough:

$ grep -lr setTestTimeout {ee/,}spec/frontend | grep _spec
ee/spec/frontend/design_management/router_spec.js
spec/frontend/clusters/clusters_bundle_spec.js
spec/frontend/gfm_auto_complete_spec.js

After the migration from Karma to Jest is done (&895 (closed)), we should take a look at these.

Side note: The current timeout for Jest is 300ms while it is 2000ms for Karma.