This introduces a feature flag, "test-require-arg", that removes builtin test's zero and one argument special modes.
That means:
- `test -n` returns false
- `test -z` returns true
- `test -x` with any other option errors out with "missing argument"
- `test foo` errors out as expecting an option
`test -n` returning true is a frequent source of confusion, and so we are breaking with posix in this regard.
As always the flag defaults to off and can be turned on. In future it will default to on and then eventually be made read-only.
There is a new FLOG category "deprecated-test", run `fish -d deprecated-test` and it will show any test call that would change in future.
