feat(test): use random ports for testing

- port is assigned by the OS.

tests/health_check.rs

@@ -1,24 +1,32 @@
+use std::{fmt::format, net::TcpListener};
 // tokio spins up a new async runtime every time 
 // at the beginning of each test case and shutdown at
 // the end of each test case
 // the spawn_app() function therefore only survives as long as the runtime
 #[tokio::test]
 async fn health_check_works(){
-    spawn_app();
+    let server_address = spawn_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.");
+    let response = client.get(&format!("{}/health_check", &server_address)).send().await.expect("Failed to execute health_check request.");
 
     assert!(response.status().is_success());
     assert_eq!(Some(0), response.content_length());
 }
 
-fn spawn_app() {
-    let server = email_newsletter_api::run().expect("Failed to bind address");
+fn spawn_app() -> String {
+    let listener = TcpListener::bind("127.0.0.1:0").expect("Failed to bind to a random port");
+
+    let port = listener.local_addr().unwrap().port();
+
+    let server = email_newsletter_api::run(listener).expect("Failed to bind address");
+
     // run() returns an instance of HttpServer that will run forever.
     // We don't want this behavior
     // Therefore we want to spawn our server, run our test logic
     // and then tear down the entire test suite
     let _ = tokio::spawn(server);
+
+    format!("http://127.0.0.1:{}", port)
 }