Since commit f65f20938c (Autogen: Avoid processing CSharp targets,
2020-11-12, v3.20.0-rc1~301^2) we collect all sources for a target
earlier than previously. Clear the sources cache so that it will be
re-computed later after AUTOGEN processing.
Fixes: #22085
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.
