Make data checksum tests more resilient for slow machines

Make data checksum tests more resilient for slow machines

The test for re-running checksum enabling was only checking for the
data checksum state to transition to 'on', but didn't account for
the launcher process having had time to exit, thus getting an error
instead of the expected no-op.  Adding a pg_stat_activity check for
the launcher exiting resolves the error, verified by inducing delay
in the launcher.

Also wrap a variable only used in injection point tests within the
correct USE macros to avoid warning for an unused variable.

All per the buildfarm.

Author: Daniel Gustafsson <daniel@yesql.se>
Reported-by: Buildfarm
Discussion: https://postgr.es/m/1CB288C9-564B-4664-B096-C2F4377D17AB@yesql.se