Qt Designer is included in Qt sources and works only for UI files. Qt Creator (the IDE) is a separate application and a separate project. You need to install or compile it first, for example from qt.io/downloads.
Once you have Qt Creator installer, you need to open the settings, add your Qt Version, then add a so called Kit (which is Qt Version + compiler bundle), and then you are ready to go.
If you want to avoid all that hassle, use the Qt installer from the link above - it will install Qt, Qt Creator, and set everything up for you, completely automatically.
I've partialy analysed AutoTest code.
For QTest it checks if given source file (after macro preprocesing) has QTest include and QTest::exec() somewhere in code.
In my case it lead to usage of TestRunner modified to store function object
Macro is placed at begining of each CPP file with test details (to decouple as much as possible)
However Autotest still acts strangely. Sometimes it discover tests, sometimes not. I believe that it may search in source context of currently selected run target.
PS: When I expanded one of DECLARE_TEST_RUNNER macro - AutoTest start displaying green arrows. Not before. Building main exe or ut.exe isn't helping.
Calling 'Refresh test' from Tools->Tests also doesn't help. I still don't know what triggers test gathering.
PSS: In each test binary at least one QTest::qExec should be expanded out of MACRO. From this point - AutoTest shows also test runners hidden in MACRO and run then all.