Stop counting for RNG output in full_stack_target

The `full_stack` fuzzer ensures that RNG output is unique by
keeping a counter of the number of `get_secure_random_bytes` calls
and using it to determine the "random" value to return.

However, because LDK regularly changes when it requests RNG output
this causes the fuzz input required to reach a codepath to change
regularly, making any existing fuzz corpus stale.

Instead, here, we allow the fuzz input to set a new RNG output
value, but otherwise always return the same output. This allows the
fuzzer to still reach RNG-output-specifc paths, but fuzzing seeds
aren't invalidated when LDK changes.

Author: TheBlueMatt