[tests] test `encode_prompt()` in isolation (#10438)
* poc encode_prompt() tests
* fix
* updates.
* fixes
* fixes
* updates
* updates
* updates
* revert
* updates
* updates
* updates
* updates
* remove SDXLOptionalComponentsTesterMixin.
* remove tests that directly leveraged encode_prompt() in some way or the other.
* fix imports.
* remove _save_load
* fixes
* fixes
* fixes
* fixes
