use lcov in both places, and try to fix the symlink nightmare

WIP to improve coverage. It's mostly small things.

The relevant coverage info, for this branch, is at https://gitlab.inria.fr/cado-nfs/cado-nfs/-/jobs/artifacts/coverage-hacks/file/coverage/index.html?job=merge+coverage+tests

Not sure that a badge works in the comments markdown, but trying isn't hard.