Avoid time-of-day-dependent failure in log rotation test.

Buildfarm members pogona and petalura have shown a failure when
pg_ctl/t/004_logrotate.pl starts just before local midnight.
The default rotate-at-midnight behavior occurs just before the
Perl script examines current_logfiles, so it figures that the
rotation it's already requested has occurred ... but in reality,
that rotation happens just after it looks, so the expected new
log data goes into a different file than the one it's examining.

In HEAD, src/test/kerberos/t/001_auth.pl has acquired similar code
that evidently has a related failure mode.  Besides being quite new,
few buildfarm critters run that test, so it's unsurprising that
we've not yet seen a failure there.

Fix both cases by setting log_rotation_age = 0 so that no time-based
rotation can occur.  Also absorb 004_logrotate.pl's decision to
set lc_messages = 'C' into the kerberos test, in hopes that it will
work in non-English prevailing locales.

Report: https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=pogona&dt=2020-12-24%2022%3A10%3A04
Report: https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=petalura&dt=2020-02-01%2022%3A20%3A04

src/bin/pg_ctl/t/004_logrotate.pl

@@ -12,6 +12,8 @@ $node->init();
 $node->append_conf(
 	'postgresql.conf', qq(
 logging_collector = on
+# these ensure stability of test results:
+log_rotation_age = 0
 lc_messages = 'C'
 ));