Corrected app_loading test so it does not generate a key error when TZ is not set in the environment (e.g., on Windows).

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12379 bcc190cf-cafb-0310-a4f2-bffc1f526a37

tests/regressiontests/app_loading/tests.py

@@ -10,7 +10,7 @@ Test the globbing of INSTALLED_APPS.
 >>> old_sys_path = sys.path
 >>> sys.path.append(os.path.dirname(os.path.abspath(__file__)))
 
->>> old_tz = os.environ["TZ"]
+>>> old_tz = os.environ.get("TZ")
 >>> settings = Settings('test_settings')
 
 >>> settings.INSTALLED_APPS