[RFC] Add more tests for Config to properly verify it behaves the same on all drivers

Stuff that I know are missing:
- [ ] `Group.clear_raw()`/ `Value.clear()` on partial primary keys
- [ ] `.set("non-dict")` on group with partial primary keys should fail (this isn't handled consistently by different drivers - JSON thinks it's okay, Postgres errors but the error is raised by asyncpg, driver doesn't handle anything), e.g.:
```py
config.init_custom("TEST", 3)
# both of statements below should raise some common error
await config.custom("TEST", "1", "2").set("non-dict")
await config.custom("TEST", "1").set({"2": "non-dict"})
```