Requiring a .uia file is a regression and makes editing the runtime test set impossible with 2.1+

We have loads of .uip files that do not have .uia files (qt3d-runtime/tests/scenes, the Lancelot test suite somewhere under tests, etc.). As this is a fully supported case in 2.0, making 2.1 abort loading .uip files saying no .uia found is a regression and should not be allowed as per the usual Qt compatibility promises.