Testing is an important part of software development. First, it helps us to compare the written software with our expectations, and, after that is done, it helps to keep the software working in the future by detecting unwanted regressions.
As you are an intermediate developer, you will probably know a lot about testing, so we won't restate the basics; we will only have a look at the available Qt tooling and GUI-specific testing techniques, as the Qt framework is prevalently used to write GUI applications.