If the tests use Qt we need to forward the Qt major version option
as well.

Since KDEInstallDirs6 explicitly sets KDE_INSTALL_DIRS_NO_DEPRECATED to
false.

Fixes unit tests with Qt6 and identified the missing definition of the
full path variables for aliases.

Fixes a number of unit tests that rely on Qt in some form when using Qt6.

With this we are no longer skipping a bunch of tests when using Qt6.

This likely still misses forwarding the Qt option to the CMake calls of
the actual tests, but that's for a subsequent change.

There's a few modules relying on ECM_GLOBAL_FIND_VERSION being defined, so
set that. The exact value is less relevant here, it's mainly used for
deprecation warnings.

To avoid ambiguity with ECMQmlModule.cmake.

To preserve backward compatibility, we keep a version around of
ECMQMLModules.cmake that warns a user of it being moved and includes the
new file.