feat(test): health_check route

tests/health_check.rs

@@ -0,0 +1,15 @@
+#[tokio::test]
+async fn health_check_works(){
+    spawn_app().await.expect("Failed to spawn our app.");
+
+    let client = reqwest::Client::new();
+
+    let response = client.get("http://127.0.0.1:8000/health_check").send().await.expect("Failed to execute health_check request.");
+
+    assert!(response.status().is_success());
+    assert_eq!(Some(0), response.content_length());
+}
+
+async fn spawn_app() -> Result<(), std::io::Error> {
+    email_newsletter_api::run().await
+}