BUG: correctly create asset finder
MAINT: rename fixture
STY: fixes for flake8
STY: add space around assignment
MAINT: add var back to constructor
MAINT: remove unused import
MAINT: compare var with None directly
MAINT: fix merge errors
Renames zipline.utils.test_utils to zipline.testing
Adds zipline.testing.fixtures.ZiplineTestCase to manage setup and
teardown and adds mixins to define fixtures like an asset finder or
trading calendar.