[CI] Add a build-coverage target

Summary:
This new target build a coverage report of the extended tests.
Integration is still fairly limited for now and will be extended in
later diffs.

Test Plan: Run the build-coverage build plan.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6607