Integrate scala-partest-interface into scala-partest

[szeiger]

The original assumptions about classpaths and classloaders, which prompted the separation into two projects, do not hold in Scala's sbt build. scala-partest-interface always used reflective access to scala-partet but did so through it's own ClassLoader, which makes sense considering that both end up as libraryDependencies on the same classpath. There doesn't seem to be any reason to keep the two parts separate.

Review by @adriaanm

[szeiger]

Note: I got the following error at the end of a complete partest run when trying this locally:

Exception in thread "Thread-42" java.lang.NullPointerException
    at sbt.TestEvent$$anonfun$overallResult$1.apply(TestReportListener.scala:72)
    at sbt.TestEvent$$anonfun$overallResult$1.apply(TestReportListener.scala:71)
    at scala.collection.IndexedSeqOptimized$class.foldl(IndexedSeqOptimized.scala:51)
    at scala.collection.IndexedSeqOptimized$class.foldLeft(IndexedSeqOptimized.scala:60)
    at scala.collection.mutable.WrappedArray.foldLeft(WrappedArray.scala:34)
    at scala.collection.TraversableOnce$class.$div$colon(TraversableOnce.scala:138)
    at scala.collection.AbstractTraversable.$div$colon(Traversable.scala:105)
    at sbt.TestEvent$.overallResult(TestReportListener.scala:71)
    at sbt.TestEvent$$anon$1.<init>(TestReportListener.scala:66)
    at sbt.TestEvent$.apply(TestReportListener.scala:65)
    at sbt.React.react(ForkTests.scala:129)
    at sbt.ForkTests$$anonfun$mainTestTask$1$Acceptor$2$.run(ForkTests.scala:74)
    at java.lang.Thread.run(Thread.java:745)

I doubt that it was caused by these changes. I was unable to reproduce it and I don't have an explanation for it. The error points to an Event being null but scala-partest emits sbt test events only in a single location where they are guaranteed to be nun-null.

[szeiger]

I did two more runs of the whole test suite with this version and couldn't reproduce the problem.

[retronym]

LGTM. I'm going to tag a release so I can use this on the 2.12 branch, and save having to bring sbt-partest-interface into the bootstrap script.