[1.8.x] Refactored tests that rely on an ImportError for Python 3.5 compatibility

A change in Python test discovery [1] causes the old packages that raised
an error to be discovered; now we use a common directory that's
ignored during discovery. Refs #23763.

[1] http://bugs.python.org/issue7559

Backport of c0cc8f69e7 from master

tests/runtests.py

@@ -35,6 +35,7 @@ os.environ['DJANGO_TEST_TEMP_DIR'] = TEMP_DIR
 
 SUBDIRS_TO_SKIP = [
     'data',
+    'import_error_package',
     'test_discovery_sample',
     'test_discovery_sample2',
     'test_runner_deprecation_app',