Add ability to extend TestCall to create custom chained helpers

[JHWelch]

What:

• Bug Fix
• New Feature

Description:

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

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

I had this idea after discussing on Pinkary and realize this was not currently a way Pest could be extended.

This opens up the framework for a whole new area of extensibility.

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

[JHWelch]

This is required because extend is being called non-statically. With this combination no errors are thrown and no output is included from a test()->extend(...) call.

I would need to add a different StaticallyExtendable or something like it to be able to use TestCall::extend(...) which is another possible API.