rust-tests: add test to check if `Cargo.lock` is up-to-date An out-of-date `Cargo.lock` is the source of rebase conflicts, prevents a reproductible build, introduces desync between the series that introduces a change in dependencies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 This commit demonstrates that the test works, the next one fixes the currently out-of-date `Cargo.lock`.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

--- a/tests/hghave.py	Tue Sep 01 18:08:24 2020 +0530
+++ b/tests/hghave.py	Thu Sep 24 16:30:00 2020 +0200
@@ -1066,6 +1066,11 @@
     )
 
 
+@check("cargo", "cargo tool")
+def has_cargo():
+    return matchoutput('`rustup which cargo` --version', b'cargo')
+
+
 @check("lzma", "python lzma module")
 def has_lzma():
     try: