[junitlauncher] show standard output/error while running tests

Hey,

I've been trying to get the `junitlauncher` taskdef to print the output 
of tests while they're being run.

The builtin listeners (legacy-{plain,brief,xml}) only write their output 
after the tests have concluded, so as a next step I tried implementing 
my own `TestResultFormatter`, but realized that the `sysOutAvailable` 
and `sysErrAvailable` methods are only called at the end after tests, 
and not live during execution.

Is there any way to achieve this, making junitlauncher print output as 
it arrives instead of as a final summary?

Cheers,
Luna