Add test to ensure no warnings from default config.

[joejuzl]

The default config should not cause warnings.
Add a test to confirm no warnings are created from the default rasa init when training or loading a model.

Definition of done

• Ensure that the default config has no warnings for training and loading a trained model
• Add a test to ensure no warnings from default config

[TyDunn]

As part of this, we should ensure that Warning related to constrain_similarities shouldn't come up for a new rasa init project since auto-configuration already uses constrain_similarities=True afaik. and close the linked issue

[joejuzl]

This is also blocked by a warning that will be removed in #8880 (in MemoizationPolicy)