Resolves T5976. So when Powershell thinks it doesn't have an output terminal, it will automatically encode all output to standard error in a terrible CLIXML format. No amount of command-line options will prevent this from occurring (I've tried). You also can't wrap the command on the Windows side of things, because in order to do any processing on that level, you'd need to execute Powershell (which would in turn encode it again), or output all standard error to a text file and then type it from Batch (which would lose the ability to stream the standard error to Harbormaster).

Direct your anger at Microsoft for writing such terrible software and then run the unit tests.