[8.x] Fix inconsistency in table names in validator

[mvalim]

This PR fix a issue where the ValidatesAttributes::parseTable return the wrong table when the $table attribute in the model is prefixed with a schema. This PR only affects calls where the model itself is provided as argument and it has a prefixed table name like $table = 'prefix.table', so it should be backwards compatible,

I also added two assertions to try to ensure this change does not affect other scenarios.

The original issue #37580

The problem:

Schema::create('public.users', function (Blueprint $table) { }); // works

$table = 'public.users'; // works

DB::table('public.users')->first() //works

class User extends Model {
   $table = 'public.users';
}
Rule::unique(User::class) // don't work -> Database connection [public] not configured.

[GrahamCampbell]

If this is a bug fix, it should go to 6.x.

[mvalim]

Sorry for that, I'm not used to making pull requests and I don't fully understand the open source flow, I only made this pull request because I really need this to be fixed. And again sorry about my ignorance with this next question, but how should I proceed now?