rust-tests: add test to check if `Cargo.lock` is up-to-date
An out-of-date `Cargo.lock` is the source of rebase conflicts, prevents a
reproductible build, introduces desync between the series that introduces a
change in dependencies and the one that eventually commits the `Cargo.lock`,
and is just a general annoyance.
This commit demonstrates that the test works, the next one fixes the
currently out-of-date `Cargo.lock`.
Differential Revision: https://phab.mercurial-scm.org/D9079
#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