rt-app: redirect stdout to stderr

By default rt-app writes to stderr, not to stdout. Therefore, if we
only sample the stdout, we get no output from the workload itself.

With this change, we redirect stderr to stdout to allow us to get the
workload output.

wa/workloads/rt_app/__init__.py

@@ -173,6 +173,7 @@ class RtApp(Workload):
     def run(self, context):
         self.output = self.target.invoke(self.command,
                                          on_cpus=self.taskset_mask,
+                                         redirect_stderr=True,
                                          timeout=self.timeout,
                                          as_root=True)