Make WAGTAIL_ROOT point to the actual Wagtail root...

Ever since the test settings got their own file, this has pointed to
the 'tests' folder instead. Luckily this had no ill effects, since
it's only used as the base for the temporary STATIC_ROOT / MEDIA_ROOT
folders.

wagtail/tests/settings.py

@@ -2,9 +2,9 @@ from __future__ import absolute_import, unicode_literals
 
 import os
 
-WAGTAIL_ROOT = os.path.dirname(__file__)
+WAGTAIL_ROOT = os.path.dirname(os.path.dirname(__file__))
-STATIC_ROOT = os.path.join(WAGTAIL_ROOT, 'test-static')
+STATIC_ROOT = os.path.join(WAGTAIL_ROOT, 'tests', 'test-static')
-MEDIA_ROOT = os.path.join(WAGTAIL_ROOT, 'test-media')
+MEDIA_ROOT = os.path.join(WAGTAIL_ROOT, 'tests', 'test-media')
 MEDIA_URL = '/media/'
 
 DATABASES = {