"not not return 34" exits with 34, not 1. This behavior is pretty
surprising but benign. I think it's very unlikely that anyone relies
on the opposite behavior, because using two "not" decorators in one
job is weird, and code that compares not's raw exit code is rare.
The behavior doesn't match our docs, but it's not worth changing the
docs because that would confuse newcomers. Add a test to cement the
behavior and a comment to explain this is intentional.
I considered adding the comment at
parse_execution_context_t::populate_not_process where this behavior
is implemented but the field defintion seems even better, because I
expect programmers to read that first.
Closes#8377
