mUnit should be listed at the testing framworks page

[frankbe]

mUnit is the testing library of scala toolkit and it does support scala.js.
IMO it's supposed to be listed at the testing framework page, so.
Maybe even in the first place since the toolkit tools somehow can be considered as default solutions.

[sjrd]

PR welcome. ;)

[frankbe]

PR created. Against my advice, I put it second on the list as JUnit with its own plugin (ScalaJSJUnitPlugin) seems to play a special role. Feel free to change the order as needed.