Add docs for best practices of configuring a provider during testing

### Does this documentation exist?

* [ ] This is new documentation
* [x] This is an enhancement to existing documentation

### Where would you expect to find this documentation?

https://developer.hashicorp.com/terraform/plugin/testing/acceptance-tests/configuration

#### Details

There isn't a doc for configuring the provider for testing. Directly put the provider configuration block in the test config would trigger the provider re-configured warning.

A documentation for the best practices of configuring a provider, including the provider being tested and external providers, seems to be needed.

There are some ways to configure the provider during testing in HashiCorp maintained providers repository, like AWS provider: https://github.com/hashicorp/terraform-provider-aws/blob/c251ae9447a116ef51715a28e0e21204bf92450c/internal/acctest/acctest.go#L312-L333

However, adding a doc to formally describe it would be much better and easier for plugin developers to understand.