test/rbd: Improve/update rbd-ggate

test/rbd: Improve/update rbd-ggate

* check if geom_gate can be loaded before doing the actual tests

Otherwise continuing does not make sense.
Major reason for this problem is due to mismatch between
kernel and module versions.

* After FreeBSD kernevel 1200078 ggate resizing is possible

So set the flag that resizing can be tested

* Only sudo commands that really need sudo

rbd-ggate list is available in regular user mode

* be a bit more verbose during testing and list the test purpose

* list-mapped is an option in rbd-nbd, not (yet) in rbd-ggate

Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>