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)
Status Job ID Name Coverage
  Preparation
passed #53745
manifest

00:00:21

 
  Build
passed #53746
build fedora x86_64

00:02:06

 
  Full Builds
passed #53747
build android arm64 api28

00:04:00