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_hiddeninput.py

@@ -0,0 +1,10 @@
+from django.forms import HiddenInput
+
+from .base import WidgetTest
+
+
+class HiddenInputTest(WidgetTest):
+    widget = HiddenInput()
+
+    def test_render(self):
+        self.check_html(self.widget, 'email', '', html='<input type="hidden" name="email" />')