Rewrote form widget tests as proper unittests.

This is preparation for landing the template-based widget rendering
patch and goes a long way to making these tests more useful for future
development. The old doctest heritage is strong here.

tests/forms_tests/widget_tests/test_fileinput.py

@@ -0,0 +1,16 @@
+from django.forms import FileInput
+
+from .base import WidgetTest
+
+
+class FileInputTest(WidgetTest):
+    widget = FileInput()
+
+    def test_render(self):
+        """
+        FileInput widgets never render the value attribute. The old value
+        isn't useful if a form is updated or an error occurred.
+        """
+        self.check_html(self.widget, 'email', 'test@example.com', html='<input type="file" name="email" />')
+        self.check_html(self.widget, 'email', '', html='<input type="file" name="email" />')
+        self.check_html(self.widget, 'email', None, html='<input type="file" name="email" />')