Re-Arrange structure. Add unit testing.

tests/health_check.rs

@@ -0,0 +1,30 @@
+use std::net::TcpListener;
+
+#[tokio::test]
+async fn health_check_works() {
+    // Arrange
+    let address = spawn_app();
+    let client = reqwest::Client::new();
+
+    // Act
+    let response = client
+        .get(&format!("{}/health_check", &address))
+        .send()
+        .await
+        .expect("Failed to execute request");
+
+    // Assert
+    assert!(response.status().is_success());
+    assert_eq!(Some(0), response.content_length());
+}
+
+fn spawn_app() -> String {
+    let listener = TcpListener::bind("127.0.0.1:0")
+        .expect("Failed to bind to random port.");
+    let port = listener.local_addr().unwrap().port();
+    let server = mail_app::run(listener).expect("Failed to bind address");
+    // Launch in background
+    let _spawn = tokio::spawn(server);
+
+    format!("http://127.0.0.1:{}", port)
+}