Tests for YAML stuff

YAML is currently disabled in tests, due to a bug in ReSharper being unable to process notifications that the YAML PSI module is available. This means we have no tests for the features built on top of the YAML PSI 🙀

* PSI Module creation
* Meta file cache
* YAML references in `.asset`, `.prefab`, `.unity`
* Usage suppression of event handlers + MB classes based on YAML references
* Find usages
* Grouping of usages in Rider (not sure this is actually testable)
* Rename (assert disabled, test working when implemented)