qtmultimedia's tst_qcamera conflicts with one in qt3d

The fact that two tests have the same name causes a cmake error. (Of course we probably didn't care when we were using qmake, but now one can type "ninja tst_something" after successful configuration of all of qt6, to rebuild one autotest and all its deps, without giving a path. So clearly it expects executable names to be unique.) I have to avoid building qt3d for now. I guess this sort of thing is going to recur whenever test executable names are too vague.