Tests don't run with full coverage

[chrysn]

Right now, tests are done with selected boards inside GitHub runners.

The nicer way would be to trigger Murdock builds; road there is roughly to replace the current build action with one that:

• clones both current master and the latest release branch,
• fabricates a commit on top of them that replaces records in .cargo/config.toml (on the release branch: creates) and the Cargo.lock files that bends the riot-wrappers reference to the branch and commit under test,
• pushes both these commits to a public location (eg. https://git.riot-os.org/), and
• triggers Murdock to evaluate these versions of RIOT.

As the last step is blocked on having the new Murdock front-end in place, this endeavour is delayed until then.

All this relates likewise to riot-sys -- keeping this as an issue for both until paths diverge.