apertis_test_lib: Allow setting discoverable & pairable without a callback

Sometimes just setting the adaptor to be pairable and/or discoverable
as a one-shot action without change notifications is enough. Support
this.

Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D2202