take testMaibox setting from akka.testMailbox system property

also improve doc comment in reference.conf

akka-actor/src/main/resources/reference.conf

@@ -254,7 +254,8 @@ akka {
 
     debug {
       # enable function of Actor.loggable(), which is to log any received message at
-      # DEBUG level, see “Testing Actor Systems”
+      # DEBUG level, see the “Testing Actor Systems” section of the Akka Documentation
+      # at http://akka.io/docs
       receive = off
 
       # enable DEBUG logging of all AutoReceiveMessages (Kill, PoisonPill and the like)

build.sbt

@@ -6,4 +6,3 @@
 
   (description in LsKeys.lsync) := "Akka is the platform for the next generation of event-driven, scalable and fault-tolerant architectures on the JVM."
 
-  //testMailbox in Global := true

project/AkkaBuild.scala

@@ -26,7 +26,7 @@ object AkkaBuild extends Build {
     id = "akka",
     base = file("."),
     settings = parentSettings ++ Release.settings ++ Unidoc.settings ++ Rstdoc.settings ++ Publish.versionSettings ++ Dist.settings ++ Seq(
-      testMailbox in GlobalScope := false,
+      testMailbox in GlobalScope := System.getProperty("akka.testMailbox", "false").toBoolean,
       parallelExecution in GlobalScope := System.getProperty("akka.parallelExecution", "false").toBoolean,
       Publish.defaultPublishTo in ThisBuild <<= crossTarget / "repository",
       Unidoc.unidocExclude := Seq(samples.id, tutorials.id),