This issue would only manifest when a custom task runner was being used with
a custom compositor. Both were tested separately but not together. A new
test has been added for this. We still create the GPU thread merger
unnecessarily but I can patch that later. I also cleaned up the existing
custom task runner test to not submit tasks on a dead engine as they just
log errors unnecessarily.

Filed new: https://github.com/flutter/flutter/issues/38844