Use tempdir_short instead of tempdir.

After cd64dc42d1, a significant
percentage of the buildfarm got unhappy, because pg_basebackup chokes
if it tries to create a tarfile with symlink more than 99 characters
in length. To try to fix that problem, use tempdir_short instead of
tempdir, as we do in pg_verifybackup's 003_corruption.pl.

There's a more complicated workaround for the same issue in
pg_basebackup's 010_pg_basebackup.pl, but I'm not clear whether
there's any reason to do it that way here. For now, let's try this,
to at least get the buildfarm green again.

A better long-term fix would be to figure out how to generate tar
files containing long symlinks, but that will have to wait for
another time.

src/bin/pg_combinebackup/t/002_compare_backups.pl

@@ -7,7 +7,7 @@ use PostgreSQL::Test::Cluster;
 use PostgreSQL::Test::Utils;
 use Test::More;
 
-my $tempdir = PostgreSQL::Test::Utils::tempdir;
+my $tempdir = PostgreSQL::Test::Utils::tempdir_short();
 
 # Set up a new database instance.
 my $primary = PostgreSQL::Test::Cluster->new('primary');