Require test coverage to be kept same or increased

### What problem are you facing?

Upjet is a project that is depended by official providers, which will be used by most of our customers in production. Today, we have good coverage of unit tests but there are cases here and there that we don't.

### How could Terrajet help solve your problem?

In Crossplane, we have the [coverage change reported](https://github.com/crossplane/crossplane/blob/master/.github/workflows/ci.yml#L169) and it fails if the PR decreases the coverage. We should have the same here and make it required to pass to keep the quality of our software high.