feat(test/polyrith): better formatting for generated tests (#15728)

The `polyrith` test suite is unusually structured to avoid making dozens of calls to Sage. A helper tactic calls `polyrith` and saves the output from Sage along with intermediate information, printing a full test as a "Try this:". This PR makes that helper tactic format its output better, spread over multiple lines as requested at #15292 (comment) by @eric-wieser .



Co-authored-by: Rob Lewis <rob.y.lewis@gmail.com>