add snippets about terraform tests

https://developer.hashicorp.com/terraform/language/tests

terraform/tests/main.tf

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+provider "aws" {
+  region = "ap-northeast-1"
+}

terraform/tests/outputs.tf

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+output "bucket_name" {
+  value = aws_s3_bucket.bucket.bucket
+}

terraform/tests/s3.tf

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+resource "aws_s3_bucket" "bucket" {
+  bucket = "${var.bucket_prefix}-bucket"
+}

terraform/tests/s3.tftest.hcl

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+variables {
+  bucket_prefix = "test"
+}
+
+run "uses_root_level_value" {
+
+  command = plan
+
+  assert {
+    condition     = aws_s3_bucket.bucket.bucket == "test-bucket"
+    error_message = "S3 bucket name did not match expected"
+  }
+
+}
+
+run "overrides_root_level_value" {
+
+  command = plan
+
+  variables {
+    bucket_prefix = "other"
+  }
+
+  assert {
+    condition     = aws_s3_bucket.bucket.bucket == "other-bucket"
+    error_message = "S3 bucket name did not match expected"
+  }
+
+}

terraform/tests/variables.tf

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+variable "bucket_prefix" {
+  type = string
+}