Rewrite coverage.sh to use with_coverage.

coverage.sh is now a more elaborate script to before, thanks to moving from tarpaulin to grcov via with_coverage.sh.

This commit also adds support for the minimalistic chutney+curl-based integration test from our gitlab-ci.yml file.

See the message from "coverage.sh -h" for information about running these tests.

Closes #248 (closed).