tests: Use the right Perl operator

We were using != to compare strings, for which "ne" is the right thing.
It's not clear why it works everywhere except on Pavan's machine, but
it's clearly bogus anyway.

Author and reporter: Pavan Deolasee
Discussion: https://postgr.es/m/CABOikdPhsHM+pX8skoEY1_T0OtKdO1udzUj4VCjU5VEt+bj4eA@mail.gmail.com

src/test/modules/commit_ts/t/004_restart.pl

@@ -54,7 +54,7 @@ my $xid = $node_master->safe_psql(
 
 my $before_restart_ts = $node_master->safe_psql('postgres',
 	qq[SELECT pg_xact_commit_timestamp('$xid');]);
-ok($before_restart_ts != '' && $before_restart_ts != 'null',
+ok($before_restart_ts ne '' && $before_restart_ts ne 'null',
 	'commit timestamp recorded');
 
 $node_master->stop('immediate');