This makes the js batched test runner cooperate with emrun.
The runner sends back the output and exit messages to emrun to inform it
about the test execution state.
The code is based on emrun_postjs.js from the emsdk.
Change-Id: I758f2c185797f4000810eb4314423eebc1c5d457
Reviewed-by: David Skoland <david.skoland@qt.io>
