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

tests/test-check-cargo-lock.t

@@ -8,4 +8,4 b' Check if Cargo.lock is up-to-date. Will '
 
 However most CIs will run `cargo build` or similar before running the tests, so we need to check if it was modified
 
-  $ testrepohg diff
+  $ testrepohg diff Cargo.lock