Commit 7132810c (Retain tempdirs for failed tests) used Test::More's
is_passing method, but that was added in Test::More 0.89_01 which is
sometime later than Perl 5.10.1.  Popular platforms such as RHEL6 don't
have that, nevermind some of our older dinosaurs.  Do it the hard way.

Michael Paquier, based on research by Craig Ringer