[2.x] Test helper for partial requests

[pascalbaljet]

Inspired by #595.

Inertia::render('page', [
    'foo' => 'bar',
    'lazy' => Inertia::lazy(fn () => 'baz'),
])

//

$response->assertInertia(function ($inertia) {
    $inertia->where('foo', 'bar');
    $inertia->missing('lazy');

    $inertia->partial('lazy', function ($inertia) {
        $inertia->where('lazy', 'baz');
        $inertia->missing('foo');
    });
});

Maybe we can add options for only and except, and add a reload method as well.

[pascalbaljet]

Inertia::render('page', [
    'foo' => 'bar',
    'lazy1' => Inertia::lazy(fn () => 'baz'),
    'lazy2' => Inertia::lazy(fn () => 'qux'),
]);

//

$response->assertInertia(fn ($page) => $page
    ->where('foo', 'bar')
    ->missing('lazy1')
    ->missing('lazy2')
    ->reloadOnly('lazy1', fn ($reload) => $reload
        ->missing('foo')
        ->where('lazy1', 'baz')
        ->missing('lazy2')
    )
);

Renamed partial to reloadOnly and added reloadExcept and reload as well.