TST: Use tempfiles in all tests.
related #5419

Includes @jreback's commits from #5422 and hdf_temp:
- TST: make pytables tests go thru a temporary dir and file
- TST/BUG: incorrect way of testing for r+ modes

TST: fix temporary files by using mktemp (rather than mkstemp) which opens them

Why don't we just not use ensure_clean for hdf? Already has a custom
function anyways, could just create temp dir with mktemp in setUp then
remove in tearDown (module level)
 don't actually use the ensure_clean very much, just a couple of time, it DOES have a custom one (that generates stores).....anyway fixed already
 @jtratner ok now?
 @jtratner ok?
 Yes, nice and principled :)
