Write OpenTofu tests for SMTP brokerpak

[jameshochadel]

Once https://github.com/cloud-gov/product/issues/2988 is working and informally tested, write tests for the brokerpak using OpenTofu's testing feature.

• OpenTofu docs on tofu test: https://opentofu.org/docs/cli/commands/test/
• Reference from data.gov: https://github.com/GSA-TTS/terraform-cloudgov/pull/48/files