Add browser assertion helpers for available fields

[bmoex]

Since latest (patch) release our dusk tests failed cause we used ->assertChecked() and the boolean field now uses labels instead of input fields.

It would be nice to extend the FormComponent with helper assertions or maybe create a generic trait InteractsWithFields that contains most helper functions.

This would make changing the interaction/templating more flexible for development if changes happen.

I solved this now with the following assertion:

public function assertBooleanField(string $selector, bool $value): self
{
    return $this->assertAttribute($selector, 'checkedvalue', $value ? 'true' : 'false');
}

See: https://github.com/eur-rsm/laravel-nova-testing