unit/test-gatt: add support for post-tests
unit/test-gatt: add support for post-tests

Sometimes a check must be made that something happened after all data is
transferred.  A post function on the test step can check.

mapfau/bluez,mapfau/bluez,pstglia/external-bluetooth-bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez,silent-snowman/bluez,silent-snowman/bluez,pkarasev3/bluez,mapfau/bluez,mapfau/bluez,ComputeCycles/bluez,pkarasev3/bluez,ComputeCycles/bluez,pkarasev3/bluez,pstglia/external-bluetooth-bluez,pkarasev3/bluez,ComputeCycles/bluez,ComputeCycles/bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez