cargo xtask openapi generate should warn users about tests that need updating

[andrewjstone]

Specifically, after adding an endpoint to an external Nexus API, test_unauthorized_coverage will fail. It needs to be re-run with EXPECTORATE=overwrite. Warning will prevent users from finding out from CI and waiting for another round trip.

While we could automatically run this test from within cargo xtask openapi generate it would significantly slow down that operation as it would require building nexus and running the test even for changes that are not to nexus.

[davepacheco]

Sorry I hadn't dug in when we were discussing in chat.

It doesn't seem like using expectorate here is usually correct. It seems like the more common case should be adding to the unauthorized test, right? The tool could still prompt you to go update that test.

[davepacheco]

Related: #9747, #9748