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An out-of-date `Cargo.lock` is the source of rebase conflicts, prevents a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currently out-of-date `Cargo.lock`.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

tests/test-check-cargo-lock.t

@@ -0,0 +1,11 b''
+#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

tests/hghave.py

@@ -1066,6 +1066,11 b' def has_rustfmt():'
     )
 
 
+@check("cargo", "cargo tool")
+def has_cargo():
+    return matchoutput('`rustup which cargo` --version', b'cargo')
+
+
 @check("lzma", "python lzma module")
 def has_lzma():
     try: