greybus: loopback_test: make output to csv file a parameter option

Its useful to get a CSV output on stdout for test frameworks to read
and parse the results. However, a csv file is not always needed.

Add the -z option to  create/append a csv file only when the user
asks for it.

Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>