pg_validatebackup: Adjust TAP tests to undo permissions change.

It may be necessary to go further and remove this test altogether,
but I'm going to try this fix first. It's not clear, at least to
me, exactly how this is breaking buildfarm members, but it appears
to be doing so.

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

@@ -80,6 +80,7 @@ my @scenario = (
 	{
 		'name' => 'open_directory_fails',
 		'mutilate' => \&mutilate_open_directory_fails,
+		'cleanup' => \&cleanup_open_directory_fails,
 		'fails_like' => qr/could not open directory/,
 		'skip_on_windows' => 1
 	},
@@ -245,6 +246,15 @@ sub mutilate_open_directory_fails
 	return;
 }
 
+# restore permissions on the unreadable directory we created.
+sub cleanup_open_directory_fails
+{
+	my ($backup_path) = @_;
+	my $pathname = "$backup_path/pg_subtrans";
+	chmod(0700, $pathname) || die "chmod $pathname: $!";
+	return;
+}
+
 # Create a directory that can't be searched. (This is skipped on Windows.)
 sub mutilate_search_directory_fails
 {