use a 2.13-specific partest version with no scala-xml dependency
use a 2.13-specific partest version with no scala-xml dependency

I have verified in advance that this fixes the bootstrap, which was
failing since we ditched scala-xml from this repo

scala/scala,lrytz/scala,lrytz/scala,scala/scala,lrytz/scala,martijnhoekstra/scala,scala/scala,lrytz/scala,martijnhoekstra/scala,martijnhoekstra/scala,lrytz/scala,scala/scala,martijnhoekstra/scala,scala/scala,lrytz/scala,martijnhoekstra/scala,scala/scala,martijnhoekstra/scala