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

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