test_rust.sh should detect when it's being invoked improperly and error out
While attempting to test #26258 (moved), I noticed that running src/test/test_rust.sh from the top of the source tree exited with status 0 and no output. It should probably detect that it failed to find any Cargo.toml files and exit with a failure status with an error message. (This seems to happen because some necessary environment variables aren't set.)
Prior to #26258 (moved), the find invocation failing would probably have taken care of this, so this change should probably get back ported to the same releases.