Fix qt-cmake-standalone-test to work when reconfiguring a test

After successfully configuring a test using qt-cmake-standalone-test, a subsequent reconfiguration (touching the cmakelists.txt file for example) will fail, due to us modifying the CMAKE_INSTALL_PREFIX so we don't try to install the test into either the system or into Qt, and as a result the StandaloneTestsConfig.cmake files are not found anymore, and configuration fails with a can't find qt6_process_resource error.

We need to come up with a way to modify CMAKE_INSTALL_PREFIX, while keeping reconfiguration working.