We'll answer this based on our experience.

For Terraform Continuous Integration (CI), we use GitHub Actions with all of our modules. This works very well for us since we rely on GitHub. Then on a nightly basis, we run aws-nuke to clean up our environments, since failing tests frequently orphan resources that cost money and can conflict with other tests.

For a proper Terraform Continuous Delivery (CD) workflow, we think your best bet is to start with a SaaS solution and learn from that. Your options are Terraform Cloud, Scalr, Spacelift. Terraform CD is non-trivial to do well. You can easily stick Terraform into any pipeline, but a well-built terraform CD pipeline will have a terraform plan → planfile → approval → apply workflow. You'll need to stash the planfile somewhere and the planfile may contain secrets.