[Dexter] Add !then node
In order to exercise more control over stepping in Dexter tests, this patch
adds the `!then` node which can be used to step out of a function or exit
the current test. Unlike expect nodes, !then nodes appear as direct singular
children of a state node:
!where {lines: 10}: !then finish
The two currently available commands are "step_out" and "finish". step_out
performs a debugger "step out" command, skipping over all !wheres in the
current frame and not stepping into any lower !wheres. The finish command
ends the debugger session immediately after finishing the current step.
