Fix race condition in non-forked, parallel tests.

Non forked tests that are run in parallel groups can call into
a single instance of TestStatusReporter concurrently.

This seems to be limited to the startGroup/endGroup/testEvent
methods called in:

  a41727fb17/testing/src/main/scala/sbt/TestFramework.scala (L100-L124)

Which itself is called within:

  a41727fb17/testing/src/main/scala/sbt/TestFramework.scala (L203-L229)

Creating the `runnables` that are run in parallel (in builds so configured):

  a6eb1260c8/main-actions/src/main/scala/sbt/Tests.scala (L222-L230)

We believe this to be the cause of the hang witnessed in
the a suite of Scalacheck-framework tests in the Scala
build:

  https://github.com/scala/scala-jenkins-infra/issues/249

This commit uses a concurrent map to support concurrent
status updates.

main-command/src/main/scala/sbt/internal/server/Server.scala

@@ -63,7 +63,8 @@ private[sbt] object Server {
                 val maxSocketLength = new UnixDomainSocketLibrary.SockaddrUn().sunPath.length - 1
                 val path = socketfile.getAbsolutePath
                 if (path.length > maxSocketLength)
-                  sys.error("socket file absolute path too long; " +
+                  sys.error(
+                    "socket file absolute path too long; " +
                       "either switch to another connection type " +
                       "or define a short \"SBT_GLOBAL_SERVER_DIR\" value. " +
                       s"Current path: ${path}")

testing/src/main/scala/sbt/TestStatusReporter.scala

@@ -8,14 +8,16 @@
 package sbt
 
 import java.io.File
-import sbt.io.IO
 
-import scala.collection.mutable.Map
+import sbt.io.IO
 import sbt.protocol.testing.TestResult
+import java.util.concurrent.ConcurrentHashMap
+
+import scala.collection.concurrent
 
 // Assumes exclusive ownership of the file.
 private[sbt] class TestStatusReporter(f: File) extends TestsListener {
-  private lazy val succeeded = TestStatus.read(f)
+  private lazy val succeeded: concurrent.Map[String, Long] = TestStatus.read(f)
 
   def doInit = ()
   def startGroup(name: String): Unit = { succeeded remove name }
@@ -32,13 +34,16 @@ private[sbt] class TestStatusReporter(f: File) extends TestsListener {
 
 private[sbt] object TestStatus {
   import java.util.Properties
-  def read(f: File): Map[String, Long] = {
+  def read(f: File): concurrent.Map[String, Long] = {
     import scala.collection.JavaConverters._
     val properties = new Properties
     IO.load(properties, f)
-    properties.asScala map { case (k, v) => (k, v.toLong) }
+    val result = new ConcurrentHashMap[String, Long]()
+    properties.asScala.iterator.foreach { case (k, v) => result.put(k, v.toLong) }
+    result.asScala
   }
-  def write(map: Map[String, Long], label: String, f: File): Unit = {
+
+  def write(map: collection.Map[String, Long], label: String, f: File): Unit = {
     val properties = new Properties
     for ((test, lastSuccessTime) <- map)
       properties.setProperty(test, lastSuccessTime.toString)