modetest: add an add_property_optional variant that does not print errors

As new features are added and others are declared to be legacy, it's
nice to be able to implement fallbacks. As such, create a
property-setting variant that does not generate errors which can very
well be entirely expected.

Will be used for gamma control in a future change.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>