Backpatch addition of wait_for_log(), pump_until().

These were originally introduced in a2ab9c06ea and a2ab9c06ea, as they are
needed by a about-to-be-backpatched test.

Discussion: https://postgr.es/m/20220413002626.udl7lll7f3o7nre7@alap3.anarazel.de
Backpatch: 10-14

src/test/perl/PostgreSQL/Test/Utils.pm

@@ -27,6 +27,7 @@ our @EXPORT = qw(
   system_log
   run_log
   run_command
+  pump_until
 
   command_ok
   command_fails