Add the ability to extend TestCall

[JHWelch]

What:

• Bug Fix
• New Feature

Description:

This is another go at the change in #126. I know you closed that getting ready for V4.

This feels like the missing piece of enabling making really powerful Pest extensions that can just be chained off of existing tests.

Adds the ability to ->extend(...) TestCall in the same fashion that expectations are extendable.

test()->extend('fooBar', function () {
    return $this->with([
        'foo',
        'bar',
    ]);
});

If this is something you want to add, I can PR the docs repo wherever makes sense.

If you don't think you'll ever have an appetite for this let me know and I won't PR again.