tmf: lttngControl: Separate STDERR from output and create errorOutput

tmf: lttngControl: Separate STDERR from output and create errorOutput

Refactoring of current function to adjust to change.
Modifications to test scenarios to support new error output

Change-Id: I457aeed7ae6aba1ce880339c5a9b7f55c6a0e232
Signed-off-by: Jonathan Rajotte Julien <jonathan.rajotte-julien@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/31228
Tested-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>