Make reporter tests more reliable. (#3381)
Make reporter tests more reliable. (#3381)

* Make reporter tests more reliable.

The reporter tests use a global view where stats are aggregated. Across tests, that view is shared and in itself eventually consistent. Adding a retry to the check logic effectively accounts for that eventual consistency and makes the tests pass reliably.

Also:
- Refactor of the test functions to have less cyclomatic complexity.
- Refactor of the error messages to conform the got/want schema.

* Fix review comments.

knative/serving,knative/serving