Build a coverage-enabled version of the tools

This allows the offline tests to also generate coverage data, and the
hs-coverage target is changed to show both unit-test and offline-tests
coverage.

The downside is that now we build yet-another-binary, which makes the
unit-test time slower…

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>