Seperate into multiple files. Add configuration options

tests/health_check.rs

@@ -1,11 +1,12 @@
 use std::net::TcpListener;
+use mail_app::startup::run;
 
 // Create new instance of the application on a random port and return address [`http://localhost:XXXX`]
 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");
+    let server = run(listener).expect("Failed to bind address");
 
     let _ = tokio::spawn(server);