ssh: Flush writing end of pipes before closing

When running a background command, ensure the redirection thread flushes
the writing end of the pipe before closing it.

devlib/utils/ssh.py

@@ -678,6 +678,7 @@ class SshConnection(SshConnectionBase):
             # Make sure the writing end are closed proper since we are not
             # going to write anything anymore
             for r, w in out_streams.values():
+                w.flush()
                 if r is not w and w is not None:
                     w.close()