Describe Terraform's 'terraform test' framework. How does it differ from Terratest, and how would you implement a comprehensive testing strategy for a Terraform module that provisions a VPC with public/private subnets?

# tests/vpc_basic.tftest.hcl
run "validate_vpc_creation" {
  command = plan

  variables {
    vpc_cidr    = "10.0.0.0/16"
    environment = "test"
    azs         = ["us-east-1a", "us-east-1b"]
  }

  assert {
    condition     = aws_vpc.main.cidr_block == "10.0.0.0/16"
    error_message = "VPC CIDR does not match expected value"
  }

  assert {
    condition     = length(aws_subnet.private) == 2
    error_message = "Expected 2 private subnets for 2 AZs"
  }

  assert {
    condition     = length(aws_subnet.public) == 2
    error_message = "Expected 2 public subnets for 2 AZs"
  }
}

run "verify_nat_gateway" {
  command = plan

  variables {
    vpc_cidr       = "10.0.0.0/16"
    environment    = "test"
    azs            = ["us-east-1a", "us-east-1b"]
    enable_nat_gw  = true
  }

  assert {
    condition     = aws_nat_gateway.main[0].subnet_id == aws_subnet.public[0].id
    error_message = "NAT Gateway must be in a public subnet"
  }
}

run "full_apply_test" {
  command = apply

  variables {
    vpc_cidr    = "10.99.0.0/16"  # Use non-overlapping CIDR for test
    environment = "tftest"
    azs         = ["us-east-1a"]
  }

  assert {
    condition     = output.vpc_id != ""
    error_message = "VPC ID output must not be empty"
  }

  assert {
    condition     = length(output.private_subnet_ids) > 0
    error_message = "Must output at least one private subnet ID"
  }
}

# Run tests
terraform test -verbose

# Terratest equivalent (Go)
# func TestVPCModule(t *testing.T) {
#   terraformOptions := terraform.WithDefaultRetryableErrors(t, &terraform.Options{
#     TerraformDir: "../",
#     Vars: map[string]interface{}{"vpc_cidr": "10.99.0.0/16", "environment": "test"},
#   })
#   defer terraform.Destroy(t, terraformOptions)
#   terraform.InitAndApply(t, terraformOptions)
#   vpcID := terraform.Output(t, terraformOptions, "vpc_id")
#   assert.NotEmpty(t, vpcID)
# }