The minimum CMake version for Qt6 is 3.16, so all the calls to
cmake_minimum_required() are updated here to enforce that
minimum. This will avoid any CMake version-related warnings
from Qt.
Avoid hard-coding Qt5 where the tests could now be using
Qt5 or Qt6.
Fixes: #22188
Build the GAT project with the same generator as the rest of the test.
This was accidentally left out of commit 8c8731b422 (Autogen: Add test
for CMAKE_GLOBAL_AUTOGEN/RCC_TARGET, 2018-11-11, v3.14.0-rc1~396^2~2).
Some AUTOGEN tests require the Qt core libraries only and
some require the Qt gui libraries to function.
This replaces the AutogenTest.cmake script with two specific
AutogenCoreTest.cmake and AutogenGuiTest.cmake scripts that
are included on demand.
