This test runs the `Tests/QtAutogen/MocInclude` test inside symlinked
directories.  The `MocInclude` test previously relied on
`get_filename_component(... REALPATH)` to get the real source tree
location and compute the path to `AutogenCoreTest.cmake` from it.
However, this does not work on Windows due to issue #17206.  The
test has been passing on Windows only on machines where symlinks
cannot be created and the main part of the test is skipped.  On
machines where symlinks can be created, the test failed with that
approach.  Fix it by explicitly passing the path to the helper
script in as a cache entry.  Avoid relying on `REALPATH`.

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