Like in #58, we furthermore need tests to check that the should_invalidate functions of all the widgets work.

We should create a little framework for easily adding new tests, so its not a burden.

A test should specify a widget, fedmsg message-id from datagrepper.. and an output (true or false).

The message-ids should be retrieved live from datagrepper during the test run, but they should be cached behind the scenes with the python-vcr module.