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.
This is the last remnant of the old percent expansion.
It has the downsides of it, in that it is annoying to combine with
anything:
```fish
echo %self/foo
```
prints "%self/foo", not fish's pid.
We have introduced $fish_pid in 3.0, which is much easier to use -
just like a variable, because it is one.
If you need backwards-compatibility for < 3.0, you can use the
following shim:
```fish
set -q fish_pid
or set -g fish_pid %self
```
So we introduce a feature-flag called "remove-percent-self" to turn it
off.
"%self" will simply not be special, e.g. `echo %self` will print
"%self".
