Fix backstop in gin test if injection point is not reached

Fix backstop in gin test if injection point is not reached

Per Tom Lane's observation that the test got stuck in infinite loop if
the injection_points module was not loaded. It was supposed to give up
after 10000 iterations, but the backstop was broken.

Discussion: https://www.postgresql.org/message-id/2498595.1710511222%40sss.pgh.pa.us