Introduction to specific 'checks' test methods

[Solido]

Checks from dart labs.
https://pub.dev/packages/checks

Introduce some ideas on how to make signals testable.

This is open for comment and right now is not exported since it's a specific libs.
User would have to import it directly.

test() {
 final s = signal(100);
 check(s).peeked(100);
}