Use SBT's built-in JUnitXmlReportPlugin

Backporting https://github.com/playframework/playframework/pull/3152 to 2.3.x

This fixes two issues:
* Test times in junit XML reports are reported properly instead of just time="0.0"
* Test report file names include the package name so that if you have two classes with the same name in different packages they don't collide

I was more conservative with this one and didn't actually delete JUnitXmlTestListener.scala as I did on master so that folks can still use it instead of SBT's built-in one if they want. For everyone else they'll get some bug fixes