grepping for johndoe shows one match intests/regressiontests/utils/simplelazyobject.py, where the test case creates a
johndoe user.
I suspect the error comes from using unittest.TestCase instead of
django.test.TestCase. The latter does some cleanup and reinitialization after
each testcase, which is necessary, but isn't happening here. Fixing the import
line should do the trick.