Ensure ConfigureProvider Response Diagnostics are Unit Tested

[bflad]

terraform-plugin-mux version

v0.11.1

Use cases

The tf5muxserver and tf6muxserver implementations of ConfigureProvider RPC includes logic which handles diagnostics from the underlying servers.

Proposal

Implement additional unit testing for that RPC, similar to #177. The TestServer implementation can be updated to include additional response fields:

type TestServer struct {
  // ...
  ConfigureProviderResponse *tfprotov5.ConfigureProviderResponse
}

And additional RPC tests can be added to ensure those underlying server diagnostics are correctly in the final response.

Bonus: While in the area, it might be good to update StopProviderError to StopProviderResponse instead for consistency.

[github-actions]

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