Offload test coverage report to CI artifact

Recently, each test job on CI reports the test coverage after the test finishes. The list of per-function coverage is massive. It could be up to 8k lines. Although the code coverage logs are collapsed by default, Gitlab platform ignores that notation when the total line of logs is too big. As a result, it's usually hard to look for failed tests. There are so many noises in the raw logs.

That list of test coverage is arguably useful, given we do report per-package coverage. This commit offloads those logs to job artifacts instead of dumping them directly to stdout. They can be browsed directly from the artifacts.

Before	After
Sample failed job	Sample coverage report