tests: only diff the `Cargo.lock` in `test-check-cargo-lock.t`
While not perfect (you could be running tests while developing with a justified
modification to `Cargo.lock`), that use-case is small enough that I don't think
it's really going to be an issue.
This stops the test from failing for unrelated changes when running it locally.
Differential Revision: https://phab.mercurial-scm.org/D9170