[cmake] name clash between qt3d and qtquick3d - assimp target

[cmake] CMake Error at /home/tim/build/build-qt6-dev-clang_17-Debug/qtbase/lib/cmake/Qt6Core/Qt6CoreMacros.cmake:676 (add_executable):
[cmake]   add_executable cannot create target "assimp" because another target with
[cmake]   the same name already exists.  The existing target is a custom target
[cmake]   created in source directory
[cmake]   "/home/tim/dev/qt6-dev/qtquick3d/src/plugins/assetimporters/assimp".  See
[cmake]   documentation for policy CMP0002 for more details.
[cmake] Call Stack (most recent call first):
[cmake]   qtbase/cmake/QtExecutableHelpers.cmake:32 (_qt_internal_create_executable)
[cmake]   qtbase/cmake/QtTestHelpers.cmake:518 (qt_internal_add_executable)
[cmake]   qtbase/cmake/QtTestHelpers.cmake:101 (qt_internal_add_test)
[cmake]   qt3d/tests/manual/assimp/CMakeLists.txt:10 (qt_internal_add_manual_test)