We've changed the test suite such that messages to standard error are not redirected to /dev/null. Why did we do this? When R detects a stack imbalance in the garbage collection (a memory leak) it does not throw an error, and it does not throw a warning. Instead the R interpreter writes output directly to standard error.
Since the developers need to see these messages, we no longer hide the output to standard error. If your tests have any warnings(), they will appear at the end of running the test suite. You may hide these messages using either the call to mxRun(model, silent = TRUE) or the more generic suppressWarnings() call.