isset.t: fix: stop testing a non-universally available option
isset.t: fix: stop testing a non-universally available option

libexec/modernish/tests/isset.t:
- The -b (-o notify) shell option may not be available on shells
  compiled without job control, at least it isn't on bash compiled
  without it. Test -u (-o nounset) instead.

modernish/modernish