tests/test-check-cargo-lock.t

tests: use `testrepohg` in one more place in test-check-code.t

This is already used elsewhere in the test to access the current hg repo, and
avoids an error about the unknown `revlog-compression-zstd` when C extensions
aren't built.  The only other such error is in test-check-interfaces.py, but I
don't see a way to avoid it other than to create an empty scratch repo.

Differential Revision: https://phab.mercurial-scm.org/D9364

#require cargo test-repo
  $ . "$TESTDIR/helpers-testrepo.sh"
  $ cd "$TESTDIR"/../rust

Check if Cargo.lock is up-to-date. Will fail with a 101 error code if not.

  $ cargo check --locked --all --quiet

However most CIs will run `cargo build` or similar before running the tests, so we need to check if it was modified

  $ testrepohg diff Cargo.lock