Fix flakiness of BuildEventServiceTests

The flakiness was caused by waiting for the server to receive the second
life-cycle event, but not waiting for the ack to get to the client. In that
case, the interruption would happen before the life-cycle call returned, in
which case it threw the InterruptedException, and then the stream doesn't
exist yet, and so there's no mechanism to send anything to the server.

PiperOrigin-RevId: 170472383

src/main/java/com/google/devtools/build/lib/buildeventservice/BuildEventServiceTransport.java

@@ -174,6 +174,10 @@ public BuildEventServiceTransport(
     this.sleeper = sleeper;
   }
 
+  public boolean isStreaming() {
+    return besClient.isStreamActive();
+  }
+
   @Override
   public ListenableFuture<Void> close() {
     return close(/*now=*/false);