Fix one hint message in 002_pg_upgrade.pl

Hi,

There is one hint message in 002_pg_upgrade.pl that is not consistent with the
testing purpose.

# --check command works here, cleans up pg_upgrade_output.d.
command_ok(
    [
        'pg_upgrade', '--no-sync', '-d', $oldnode->data_dir,
...
ok(!-d $newnode->data_dir . "/pg_upgrade_output.d",
-    "pg_upgrade_output.d/ not removed after pg_upgrade --check success");
+    "pg_upgrade_output.d/ removed after pg_upgrade --check success");

The test is to confirm the output file has been removed for pg_upgrade --check while
the message here is not consistent. Attach a small patch to fix it.

Best Regards,
Hou Zhijie