[TwigComponent] Provide class/trait for testing component

[ker0x]

It would be great to provide a ComponentTestCase class or ComponentAssertionTrait trait to help testing component.

I was thinking of something similar to MailerAssertionsTrait so we can:

• get an instance of the component from its name and parameters
• assert that the output of the component is similar to an expected value
• etc ...

[weaverryan]

@kbond may have some ideas on this -it’s definitely missing

[kbond]

Yep, for twig components, this would be pretty straight forward.

I'm working on a new selector api for zenstruck/browser. Included in this PR is a proposed new package: zenstruck/dom, that extracts the selector system to be used with any html string. The following could be possible:

class MyComponentTest extends KernelTestCase
{
    use InteractsWithTwigComponents;

    public function test_render(): void
    {
        // simple
        $this->assertStringContainsString('some text', $this->renderComponent(MyComponent::class, ['some' => 'attributes']));

        // with zenstruck/dom
        $this->renderComponent(MyComponent::class, ['some' => 'attributes'])
            ->contains('some text')
            ->containsIn('h1', 'my title')
            ->fieldEquals('input', 'some value')
            ->containsIn(Selector::xpath('...'), 'something')
            // ...
        ;
    }
}