Add tip for go-sqlmock and Close

src/advanced/testing.md

@@ -105,9 +105,15 @@ func TestMockDB(t *testing.T) {
 	err = server.ReplaceDB(dialector)
 
 	//...
+
+	mock.ExpectClose()
 }
 ```
 
+:::tip
+Test servers automatically close the database in a test cleanup hook. If you are using `go-sqlmock`, this will generate an error for unexpected `Close` unless you add `mock.ExpectClose()` at the very end of your test.
+:::
+
 ## HTTP Tests
 
 You may want to write tests that simulate how a client would interact with your API through HTTP calls. In order to do so, use the test server's `TestRequest(*http.Request)` method. It will execute the request all the way from the router's `ServeHTTP()` implementation. Therefore, the request's lifecycle will be executed from start to finish.