gltestsrc: Run context query only during decide_allocation

Running the context query in _start and during the NULL->READY state transition
can fail because downstream elements might not be able to answer and thus the
source element would not be able to reuse downstream GLContext and GLDisplay.

This issue happened specifically when trying to use gltestsrc in playbin.
3 jobs for philn/gltestsrc-context in 6 minutes and 27 seconds (queued for 2 seconds)