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

src/test/modules/test_checksums/t/001_basic.pl

@@ -35,7 +35,11 @@ my $result =
 is($result, '9999', 'ensure checksummed pages can be read back');
 
 # Enable data checksums again which should be a no-op so we explicitly don't
-# wait for any state transition as none should happen here
+# wait for any state transition as none should happen here. Make sure to let
+# any running launcher finish in case it's still wrapping up.
+$result = $node->poll_query_until('postgres',
+	"SELECT count(*) = 0 FROM pg_catalog.pg_stat_activity WHERE backend_type = 'datachecksum launcher';"
+);
 enable_data_checksums($node);
 test_checksum_state($node, 'on');
 # ..and make sure we can still read/write data

src/test/modules/test_checksums/test_checksums.c

@@ -108,7 +108,9 @@ dcw_inject_startup_delay(PG_FUNCTION_ARGS)
 	PG_RETURN_VOID();
 }
 
+#ifdef USE_INJECTION_POINTS
 static uint32 db_fail = DATACHECKSUMSWORKER_FAILED;
+#endif
 
 void
 dc_modify_db_result(const char *name, const void *private_data, void *arg)