Use arbitrary config object in full_stack_target fuzzer

As we've added more and more configuration parameters which change
our behavior, we're increasingly missing coverage in the general
`full_stack_target` fuzzer. Sadly, a reachable `unwrap` slipped in
uncaught that should have been stopped by the `full_stack_target`.

Here, we update the `full_stack_target` fuzzer to read a full
config object to substantially increase coverage.

Author: TheBlueMatt