Improve testing for backend role assumption

[yhakbar]

NIT: I double-checked that these are valid configurations here:
https://opentofu.org/docs/language/settings/backends/s3/
https://developer.hashicorp.com/terraform/language/backend/s3

But it would be good to add integration testing that actually uses these so that we know when regression occurs in OpenTofu/Terraform preventing these attributes from being used.

Originally posted by @yhakbar in #3975 (comment)

New attributes were added to support role assumption configuration in #3975, but those configurations weren't accompanied by very comprehensive testing of their usage. It's important to have good coverage of features from integration with OpenTofu/Terraform, as that allows us to detect regression when bumping to new versions of OpenTofu/Terraform in our CI.