We're creating a cache of files that should speed up bootstraping CI jobs and baking it into the CI images with 992daf5fed/.ci/packer_cache.sh.
This PR utilizes that cache (that will start showing up in CI images shortly) by extracting it into the Kibana workspace and failing if it's missing for some reason.
* [ci][ftr][kbn/test] split up CI jobs
* [ci] run --assert-none-excluded in ci groups, before builds
* [ftr] improve error message when excluded tests found
* [ci] document other places the ciGroups live