Address Django security patch in test suite

See also: https://code.djangoproject.com/ticket/31710

Author: codingjoe

tests/testapp/forms.py

@@ -10,10 +10,14 @@
     ) + forms.ClearableFileInput.__bases__
 
 
+class MultipleFileInput(forms.ClearableFileInput):
+    allow_multiple_selected = True
+
+
 class UploadForm(forms.ModelForm):
     class Meta:
         model = FileModel
         fields = ("file", "other_file")
         widgets = {
-            "file": forms.ClearableFileInput(attrs={"multiple": True}),
+            "file": MultipleFileInput(attrs={"multiple": True}),
         }