Circuitbreaker spec cleanup (#27560)

Test cleanup:

 * No need to use mockito, replaced with TestProbe (side effect is that it actually also
  makes some test cases more explicit in what they expect)
 * Use matchers to get reasonable failure messages
 * Use types where it makes sense
 * Remove mockito dependency from akka-actor-tests

project/Dependencies.scala

@@ -175,7 +175,6 @@ object Dependencies {
         Test.scalatest.value,
         Test.commonsCodec,
         Test.commonsMath,
-        Test.mockito,
         Test.scalacheck.value,
         Test.jimfs,
         Test.dockerClient,
@@ -202,7 +201,8 @@ object Dependencies {
         Provided.levelDBNative,
         Test.junit,
         Test.scalatest.value,
-        Test.commonsIo)
+        Test.commonsIo,
+        Test.mockito)
 
   val clusterMetrics = l ++= Seq(Provided.sigarLoader, Test.slf4jJul, Test.slf4jLog4j, Test.logback, Test.mockito)