add unit test for config module

test/config_test.gleam

@@ -0,0 +1,18 @@
+import config
+import gleeunit
+import gleeunit/should
+
+pub fn main() -> Nil {
+  gleeunit.main()
+}
+
+// gleeunit test functions end in `_test`
+pub fn file_read_test() {
+  let assert Ok(cfg) = config.load_config()
+
+  cfg.mqtt_host
+  |> should.equal("192.168.1.11")
+
+  cfg.mqtt_user
+  |> should.equal("homeassistant")
+}