> If any of the variables in test.globs are changed by the test (e.g. 
appending to a list), then rerunning the test will not necessarily give
the same result.

This is true, but modifying the globals such that subsequent runs of the
same test can break equally affects subsequent runs of any other tests
that use that module:  such a test is already broken (unsafe to run with
other tests) to begin with, independent of the DocTestCase.tearDown issue.