Correctly handle TestCase.fixtures set to None.
Review Request #8695 — Created Jan. 31, 2017 and submitted
This change mirrors a change to Djblets I've made to properly handle when
TestCase.fixturesisNone(as opposed to not being defined).
Ran unit tests under Django 1.6 and 1.8.
Tool: Pyflakes Processed Files: reviewboard/webapi/tests/base.py Tool: PEP8 Style Checker Processed Files: reviewboard/webapi/tests/base.py