Add synchronization when sending events to sbt (fix ArrayIOBException).

Partest shows a single `Task` to sbt and does all the concurrency and
running subtasks itself, which isn't really how this should work, so sbt
doesn't expect the task to send back events concurrently:
`EventHandler.handle` adds events to an `ArrayList` when forking (see
[ForkMain][2]), and is called after each test, and if it happens on two
threads at exactly the wrong moment:

```
Caused by: sbt.ForkMain$ForkError: java.lang.ArrayIndexOutOfBoundsException: 15
	at java.util.ArrayList.add(ArrayList.java:459)
	at sbt.ForkMain$Run$2$1.handle(ForkMain.java:294)
	at scala.tools.partest.sbt.SBTRunner$$anon$1.onFinishTest(SBTRunner.scala:70)
	at scala.tools.partest.nest.SuiteRunner.runTest(Runner.scala:781)
	at scala.tools.partest.nest.SuiteRunner.$anonfun$runTestsForFiles$2(Runner.scala:788)
	at scala.tools.partest.package$$anon$2.call(package.scala:135)
```

See scala/scala#5663 ([build log][1] or the failed run).

[1]: https://scala-ci.typesafe.com/job/scala-2.12.x-validate-test/4300/consoleFull
[2]: https://github.com/sbt/sbt/blob/v0.13.13/testing/agent/src/main/java/sbt/ForkMain.java#L294

Author: gourlaysama

src/main/scala/scala/tools/partest/sbt/SBTRunner.scala

@@ -64,16 +64,18 @@ class SBTRunner(val config: RunnerSpec.Config,
     javacCmdPath = Option(javacCmd).map(_.getAbsolutePath) getOrElse PartestDefaults.javacCmd,
     scalacExtraArgs = scalacArgs,
     javaOpts = javaOpts,
-    scalacOpts = scalacOpts) {
+    scalacOpts = scalacOpts) { self =>
 
       override def onFinishTest(testFile: File, result: TestState): TestState = {
-        eventHandler.handle(new Event {
-          def fullyQualifiedName: String = testFile.testIdent
-          def fingerprint: Fingerprint = partestFingerprint
-          def selector: Selector = new TestSelector(testFile.testIdent)
-          val (status, throwable) = makeStatus(result)
-          def duration: Long = -1
-        })
+        self.synchronized {
+          eventHandler.handle(new Event {
+            def fullyQualifiedName: String = testFile.testIdent
+            def fingerprint: Fingerprint = partestFingerprint
+            def selector: Selector = new TestSelector(testFile.testIdent)
+            val (status, throwable) = makeStatus(result)
+            def duration: Long = -1
+          })
+        }
         result
       }
     }