Added test for email settings

tests/settings_test.rs

@@ -1,6 +1,6 @@
 #[test]
 fn can_default_settings_be_loaded() {
-    let settings =
+    let _settings =
         application::configuration::Settings::new(None).expect("error when loading settings");
 }
 
@@ -11,3 +11,15 @@ fn database_settings_can_be_loaded() {
     assert_eq!("version_checker_db", settings.database.connection_string);
     assert!(!settings.database.in_memory);
 }
+
+#[test]
+fn mail_settings_can_be_loaded() {
+    let settings =
+        application::configuration::Settings::new(None).expect("error when loading settings");
+    assert_eq!("username", settings.email.smtp_username);
+    assert_eq!("relay", settings.email.smtp_relay);
+    assert_eq!("password", settings.email.smtp_password);
+    assert_eq!("from@domain.tld", settings.email.from);
+
+    assert!(!settings.database.in_memory);
+}