Allow to mute std out in StandardStreamsCapturer

The TimeoutExtension spec was quite noise as it printed thread dumps
to the console.
With this change we mute the output to stdOut for this spec.

spock-core/src/main/java/org/spockframework/runtime/StandardStreamsCapturer.java

@@ -36,10 +36,6 @@ public synchronized void start() {
   private void startCapture(PrintStream originalStream, TeePrintStream teeStream, final boolean isOut) {
     if (originalStream == teeStream) return;
 
-    if (teeStream != null) {
-      teeStream.stopDelegation();
-    }
-
     StringMessagePrintStream notifyingStream = new StringMessagePrintStream() {
       @Override
       protected void printed(String message) {
@@ -72,9 +68,27 @@ public synchronized void stop() {
   private void stopCapture(PrintStream originalStream, TeePrintStream teeStream, boolean isOut) {
     if (originalStream != teeStream) return;
     if (isOut) {
-      System.setOut(teeStream.getDelegates().get(0));
+      System.setOut(teeStream.getOriginal());
     } else {
-      System.setErr(teeStream.getDelegates().get(0));
+      System.setErr(teeStream.getOriginal());
+    }
+  }
+
+  public synchronized void muteStandardStreams() {
+    if (outStream != null) {
+      outStream.muteOriginal();
+    }
+    if (errStream != null) {
+      errStream.muteOriginal();
+    }
+  }
+
+  public synchronized void unmuteStandardStreams() {
+    if (outStream != null) {
+      outStream.unmuteOriginal();
+    }
+    if (errStream != null) {
+      errStream.unmuteOriginal();
     }
   }