util/helper: fix process output getting truncated

A process that logs a lot of data before exiting could have its output
truncated because ProcessWrapper used to stop reading the pipe once the
process exits, but instead should always read to the end of the pipe.

Signed-off-by: Zachary Deibert <Zachary.Deibert@garmin.com>

Author: zachdeibert

labgrid/util/helper.py

@@ -124,7 +124,7 @@ def check_output(self, command, *, print_on_silent_log=False, input=None, stdin=
                     stdin_w = None
 
             process.poll()
-            if process.returncode is not None:
+            if process.returncode is not None and len(ready_r) == 0:
                 break
 
         if stdin_w is not None: