Add input argument to rock test: url endpoint to expect a 200 from

[sed-i]

Sometimes just building the rock isn't enough. It would be handy if we had an optional arg to test the app is ok. For example, for prometheus it could be /api/v1/status/buildinfo that should return a 200.

https://github.com/canonical/observability/blob/8e95f25f4d6fc10a93130a1d217e796d57df12c5/.github/workflows/_rock-build-test.yaml#L3-L4