Create TestLib.pm's tempdir underneath tmp_check/, not out in the open.

This way, existing .gitignore entries and makefile clean actions will
automatically apply to the tempdir, should it survive a TAP test run
(which can happen if the user control-C's out of the run, for example).

Michael Paquier, per a complaint from me

src/test/perl/TestLib.pm

@@ -114,7 +114,7 @@ sub tempdir
 {
 	return File::Temp::tempdir(
 		'tmp_testXXXX',
-		DIR => $ENV{TESTDIR} || cwd(),
+		DIR => $tmp_check,
 		CLEANUP => 1);
 }