Refs #187 - Use specific prepare job for tests

adding a new job (per browser/version), with it's own cache:key configured, downloadind the browsers (and / or unzipping and updating the cache) in parallel to the build commands and finally providing the test:*-jobs with the correct pre-loaded browser-binary via dependencies / artifacts

(See #187 (comment 383681187))

Instead of adding several jobs per browser/version with its own cache:key, I'm using one generic prepare-tests job which creates all needed artefacts. That IMO keeps things more simple.