rename PartestTask to SbtPartestTask

`PartestTask` is a very unfortunate name for this, because it already
exists in scala-partest; there is:

 - `scala.tools.partest.PartestTask extends org.apache.tools.ant.Task`
   in scala-partest, for running partest with ant
 - `scala.tools.partest.PartestTask extends sbt.testing.Task` for
   running partest with sbt

Depending on the order `scala-partest` and `scala-partest-interface` are
added to the dependencies, SBT can get the wrong one and crash...

Author: gourlaysama

src/main/scala/tools/partest/TestingInterface.scala

@@ -49,7 +49,7 @@ case class Runner(args: Array[String], remoteArgs: Array[String], testClassLoade
    *  @return an array of <code>Task</code>s
    *  @throws IllegalStateException if invoked after <code>done</code> has been invoked.
    */
-  def tasks(taskDefs: Array[TaskDef]): Array[sbt.testing.Task] = taskDefs map (PartestTask(_): sbt.testing.Task)
+  def tasks(taskDefs: Array[TaskDef]): Array[sbt.testing.Task] = taskDefs map (SbtPartestTask(_): sbt.testing.Task)
 
   /** Indicates the client is done with this <code>Runner</code> instance.
    *
@@ -62,7 +62,7 @@ case class Runner(args: Array[String], remoteArgs: Array[String], testClassLoade
  *
  * TODO: make configurable
  */
-case class PartestTask(taskDef: TaskDef) extends Task {
+case class SbtPartestTask(taskDef: TaskDef) extends Task {
   /** Executes this task, possibly returning to the client new tasks to execute. */
   def execute(eventHandler: EventHandler, loggers: Array[Logger]): Array[Task] = {
     val forkedCp    = scala.util.Properties.javaClassPath