doctest.DocFileTest inserts the test's path into the globs as "__file__",
but doctest.DocTestCase's tearDown method simply calls globs.clear(),
so that subsequent runs of the test case will not receive the same
initial globals.

This means that doctests referencing __file__ cannot be run repeatedly
without using a custom setUp that restores __file__.

I think most would expect symmetry between setUp and tearDown, and in
fact zope.testrunner operates with this false assumption when run
with the "-N" option to repeat a test N times.