Otherwise we have a lot of warnings like:
```
CMake Warning at /usr/lib64/cmake/Qt6/QtPublicWalkLibsHelpers.cmake:284 (message):
When trying to collect dependencies of target
'Constrained_Delaunay_triangulation_2', the non-existent target 'Qt6::Svg'
was encountered. This can likely be fixed by moving the find_package call
that pulls in 'Qt6::Svg' to the scope of directory
'/home/lrineau/Git/cgal-testsuite-dockerfiles/testsuite/CGAL-6.0-Ic-66/test/Triangulation_2_Demo'
or higher. This warning can be silenced by setting
QT_SILENCE_MISSING_DEPENDENCY_TARGET_WARNING to ON.
Call Stack (most recent call first):
/usr/lib64/cmake/Qt6/QtPublicWalkLibsHelpers.cmake:320 (__qt_internal_print_missing_dependency_target_warning)
/usr/lib64/cmake/Qt6Core/Qt6CoreMacros.cmake:609 (__qt_internal_collect_all_target_dependencies)
/usr/lib64/cmake/Qt6Core/Qt6CoreMacros.cmake:709 (_qt_internal_finalize_executable)
/usr/lib64/cmake/Qt6Core/Qt6CoreMacros.cmake:564:EVAL:1 (qt6_finalize_target)
test/Triangulation_2_Demo/CMakeLists.txt:DEFERRED
```
The issue was that `find_package(Qt6) ...` is required in any
sub-directory, because imported targets like `Qt6::Svg` are
local to the directory scope.
Laurent Rineau
fe68498e95