Reset static entity cache on POST requests in tests

Problem/Motivation

We already reset quite a few things when doing POST requests, like statically cached config. We didn't do it for entities because we didn't have an API for that for a long time.

Doing this should remove the need for a lot of manual cache clear calls.

Proposed resolution

Reset the static entity cache service.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet