Add extra tests to make sure that the form is safe

Previous implementations of this functionality mutated the base form
class, which broke a bunch of stuff.

I want to make sure that getting this form for one placeholder doesn’t
change other forms that have already been instantiated for other
placeholders.

Mutation is scary.

tests/app/main/test_placeholder_form.py

@@ -0,0 +1,18 @@
+from app.main.forms import get_placeholder_form_instance
+from wtforms import Label
+
+
+def test_form_class_not_mutated(app_):
+
+    with app_.test_request_context(
+        method='POST',
+        data={'placeholder_value': ''}
+    ) as req:
+        form1 = get_placeholder_form_instance('name', {}, optional_placeholder=False)
+        form2 = get_placeholder_form_instance('city', {}, optional_placeholder=True)
+
+        assert not form1.validate_on_submit()
+        assert form2.validate_on_submit()
+
+        assert str(form1.placeholder_value.label) == '<label for="placeholder_value">name</label>'
+        assert str(form2.placeholder_value.label) == '<label for="placeholder_value">city</label>'