Add test for rendering integer 0 in form input

tests/app/main/forms/test_govuk_text_input_field.py

@@ -0,0 +1,12 @@
+from flask_wtf import FlaskForm as Form
+
+from app.main.forms import GovukTextInputField
+
+
+def test_GovukTextInputField_renders_zero(client_request):
+    class FakeForm(Form):
+        field = GovukTextInputField()
+
+    form = FakeForm(field=0)
+    html = form.field()
+    assert 'value="0"' in html