feat: added diesel::r2d2::TestCustomizer

[kpagacz]

Closes #4152

Let me know if you need me to make any improvements. I am unsure about the Error mapping. The problem I solved there was:

• the Pool::builder requires a connection customizer's error to be ConnectionManager::Error, which is diesel::r2d2::Error
• the return value from begin_test_transaction() is QueryResult, which error is diesel::result::Error

Hence the mapping, but I wonder if this is even correct for diesel. Let me know if you need me to make any adjustments here and with the docs!

[weiznich]

Thanks 👍

I think the error mapping is fine, we somewhat need to return the error there and that looks like a reasonable way.

The documentation looks fine for me.