Ignore coverage HTML report output dir in tests.

Added it to the list of dirs ignored by tests/runtests.py.

tests/runtests.py

@@ -35,6 +35,7 @@ TEMP_DIR = tempfile.mkdtemp(prefix='django_')
 os.environ['DJANGO_TEST_TEMP_DIR'] = TEMP_DIR
 
 SUBDIRS_TO_SKIP = [
+    'coverage_html',
     'data',
     'requirements',
     'templates',