Add XMLUnit as a managed dependency and possibly to the test starter

[philwebb]

It might be nice to add this by default. See #9691 for an example of the current problems when using xml() in Mock MVC.

[andrew-j-cohen]

Hi @philwebb,

This is my first contribution to the project. I was looking for an appropriate place to add a unit test for this, but I'm not sure where that would be or if its even warranted. Any feedback you have is most welcome. As a long time avid user of spring boot, I'm looking forward to more contributions in the future.

Thanks!
-Andrew

[snicoll]

Duplicate of #9709