jpm test reports to console:
.console.warn: reusetab: 10 warnings or errors were logged to the platform's nsIConsoleService, which could be of no consequence; however, they could also be indicative of aberrant behavior.
How do I view these "warnings or errors"?
These errors are filtered out the test logging at the moment, because they were too verbose, unfortunately there is not way to turn this off at the moment. This is the code which is counting the errors/warnings and choosing what to filter: https://github.com/mozilla/addon-sdk/blob/a33887015f82f8498d0dfe5326407bfc7adde940/lib/sdk/test/harness.js#L464-L470
To view these errors and warnings you'll have to use the Browser Console for now.
You can use
jpm test --do-not-quitto run the tests and prevent Firefox from automatically quitting, so that you're able to inspect the Browser Console and other things before quitting.