pg_validatebackup: Use tempdir_short in TAP tests.

The buildfarm is very unhappy right now because TAP test
003_corruption.pl uses TestLib::tempdir to generate the name of
a temporary directory that is used as a tablespace name, and
this results in a 'symbolic link target too long' error message
on many of the buildfarm machines, but not on my machine.

It appears that other people have run into similar problems in
the past and that TestLib::tempdir_short was the solution, so
let's try using that instead.

src/bin/pg_validatebackup/t/003_corruption.pl

@@ -15,7 +15,7 @@ $master->start;
 
 # Include a user-defined tablespace in the hopes of detecting problems in that
 # area.
-my $source_ts_path = TestLib::tempdir;
+my $source_ts_path = TestLib::tempdir_short;
 $master->safe_psql('postgres', <<EOM);
 CREATE TABLE x1 (a int);
 INSERT INTO x1 VALUES (111);
@@ -103,7 +103,7 @@ for my $scenario (@scenario)
 
 		# Take a backup and check that it validates OK.
 		my $backup_path = $master->backup_dir . '/' . $name;
-		my $backup_ts_path = TestLib::tempdir;
+		my $backup_ts_path = TestLib::tempdir_short;
 		$master->command_ok(['pg_basebackup', '-D', $backup_path, '--no-sync',
 							'-T', "${source_ts_path}=${backup_ts_path}"],
 							"base backup ok");