Terraform tests giving warning "Variable referenced without definition"

👓 What did you see?

When modules created using the terraform-module-boilerplate run the terraform tests, the following warnings are appearing in the output:

╷
│ Warning: Variable referenced without definition
│ 
│   on tests/main.tftest.hcl line 21, in run "test_deployment":
│   21:     project = var.project
│ 
│ Variable "project" was referenced without providing a definition.
│ Referencing undefined variables within Terraform Test files is deprecated,
│ please add a `variable` block into the relevant test file to provide a
│ definition for the variable. This will become required in future versions
│ of Terraform.
│ 
│ (and one more similar warning elsewhere)
╵
tests/main.tftest.hcl... tearing down
╷
│ Warning: Variable referenced without definition
│ 
│   on tests/main.tftest.hcl line 21, in run "test_deployment":
│   21:     project = var.project
│ 
│ Variable "project" was referenced without providing a definition.
│ Referencing undefined variables within Terraform Test files is deprecated,
│ please add a `variable` block into the relevant test file to provide a
│ definition for the variable. This will become required in future versions
│ of Terraform.
│ 
│ (and one more similar warning elsewhere)
╵
tests/main.tftest.hcl... pass
╷
│ Warning: Variable referenced without definition
│ 
│   on tests/main.tftest.hcl line 2, in provider "google":
│    2:   default_labels = var.default_labels
│ 
│ Variable "default_labels" was referenced without providing a definition.
│ Referencing undefined variables within Terraform Test files is deprecated,
│ please add a `variable` block into the relevant test file to provide a
│ definition for the variable. This will become required in future versions
│ of Terraform.
│ 
│ (and 4 more similar warnings elsewhere)
╵

✅ What did you expect to see?

The tests should run cleanly without warnings.

💻 Where does this happen?

This is happening on all Terraform modules created from the boilerplate.

🔬 How do I recreate this?

📚 Any additional information?