Fix missing "use Test::More" in Kerberos.pm.

Apparently the only Test::More function this script uses is
BAIL_OUT, so this omission just results in the wrong error
output appearing in the cases where it bails out.

Seems to have been an oversight in commit 9f899562d which
split Kerberos.pm out of another script.

Author: Maxim Orlov <orlovmg@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CACG=ezY1Dp-S94b78nN0ZuaBGGcMUB6_nF-VyYUwPt1ArFqmGA@mail.gmail.com
Backpatch-through: 17

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

@@ -9,6 +9,7 @@ package PostgreSQL::Test::Kerberos;
 use strict;
 use warnings FATAL => 'all';
 use PostgreSQL::Test::Utils;
+use Test::More;
 
 our (
 	$krb5_bin_dir, $krb5_sbin_dir, $krb5_config, $kinit,