LUITK / Qt 6: Port the tests to CMake

Currently, no tests are executed in the CI for the Qt 6 build:

Running tests...
/usr/bin/ctest --force-new-ctest-process --verbose -j1
UpdateCTestConfiguration  from :/build/lomiri-ui-toolkit-1.3.5110+0~20251117133757.222+ubports24.04.2~1.gbp2004a6/build-qt6/DartConfiguration.tcl
UpdateCTestConfiguration  from :/build/lomiri-ui-toolkit-1.3.5110+0~20251117133757.222+ubports24.04.2~1.gbp2004a6/build-qt6/DartConfiguration.tcl
Test project /build/lomiri-ui-toolkit-1.3.5110+0~20251117133757.222+ubports24.04.2~1.gbp2004a6/build-qt6
Constructing a list of tests
Done constructing a list of tests
Updating test list for fixtures
Added 0 tests to meet fixture requirements
Checking test dependency graph...
Checking test dependency graph end
No tests were found!!!

The existing unit tests need to be ported to CMake/CTtest. The goal is to get the tests into a runnable (but most likely failing) state.

Once this is done, follow-up issues should be created to fix individual test cases.