Adds support for count test

[hotmeteor]

This PR adds support for a ->assertInertiaCount($key, $count) test method.

Often you want to be able to test that the correct data is being returned to the browser based on filtering criteria. This new method functions like the others with a simple key / comparison signature.

// Base

$this->get(route('locations.index'))
    ->assertInertia('Locations/Index')
    ->assertInertiaHas('locations')
    ->assertInertiaCount('locations', 5);

// Filtered

$query = http_build_query(['province' => 'ON']);

$this->get(route('locations.index')."?{$query}")
    ->assertInertia('Locations/Index')
    ->assertInertiaHas('locations')
    ->assertInertiaCount('locations', 2);

[claudiodekker]

Nice one Adam! 👍

[hotmeteor]

Sorry, just realized I should have included documentation update. Want me to do that quick?

[claudiodekker]

That'd be great, otherwise I can pick it up later, no worries :)