Convert EntityCacheTest and FieldUITestNoBundle to EntityNG

Wondering why we have EntityCacheTest and EntityTestCache...

Ah, EntityCacheTest is used in that weird EntityApiInfoTest that tests entity info altering and module enable crazyness. Should be possible to remove that and switch another of our test entities...

And FieldUITestNoBundle was added in #1986888: Field UI integration for entity types is brittle.

Note that just extending from EntityNG and using the NG storage is somewhat weird, because that means it is impossible to actually use those entity types (was already not really possible before).

Not sure if it's technically a blocker for removing the BC decorator, the BC decorator is only used for already converted entity types that still need to be accessed the old way.

But it certainly makes sense, especially given the direction I'm moving to in #2004244: Move entity revision, content translation, validation and field methods to ContentEntityInterface (Which basically makes EntityNG ContentEntityBase and will limit a number of features to ContentEntityInterface, like being able to be fieldable, translatable, ...). Will try to post an updated issue summary there ASAP.

Will RTBC this once it's green, we can still think about re-using other entity types in non-critical issues.

Yeah, I know. The problem is that I would have to change everything you're changing here again in #2004244: Move entity revision, content translation, validation and field methods to ContentEntityInterface (at least the entity classes), so this is somewhat pointless. Might be easier to just merge into that from the start. Or find a way to drop those types.

So, the BC mode is gone and the issue that I referenced is in, so this is no longer blocked and also no longer critical.

It also means we can aim for a nice solution instead :)

Those two no longer exist and/or have been converted elsewhere/otherwise.