report_go_test does not fail when there are no tests

When the test package does not contain any tests, the [report_go_test](https://github.com/knative/test-infra/blob/65144e695d277847c51c08f5008cd39f59b931e4/scripts/library.sh#L415) function reports as success the test flow still passes even when there is error.

https://knative.slack.com/archives/CA1DTGZ2N/p1589561963210900 is the discussion on Slack.

Start in scripts/library.sh at the report_go_test function, then reproduce the flow with a test package containing no tests and inspect how its result is handled. Done means the flow reports failure rather than success when the package has no tests or returns an error.

Written by the indexing model from the issue text.