support "dry_run=BOOL" for {{ag acquire_field}} and {{ag auto guide}}

The agActor has grown a dry_run option to the main ag command. this actually does exactly the right thing: all offsets are calculated and even sent to the telescope. But the MLP1 protocol allows offset commands to be ignored by sending some flag.

Support this at the iic level. Doing so will improve top-to-bottom tests.