Conformance tests for triggers

[layershifter]

Recently we had various discussions around triggers and reached the agreement on how it should work:

• No magic handling of props in triggers, props should be applied always. I.e consistently set the same attributes and props
• child.props always win, it is user's responsibility to handle it
  • The exclusion from the rule are callbacks, they are merged

Another point that we could cover that contract with conformance tests and enable them for components that use the trigger pattern. For an initial implementation the following scenarios should be covered.

Props merging

Props specified by a customer always win.

function App() {
  return (
    <Trigger>
      <button id="foo" aria-label="bar" />
    </Trigger>
  );
}

This should render following markup:

<button id="foo" aria-label="bar"></button>

Callback merging

Internal and user's callbacks should be merged.

function App() {
  return (
    <Trigger>
      <button onClick={() => console.log('foo') />
    </Trigger>
  );
}

• internal onClick handler in Trigger should be called if present
• onClick specified by user should be also called

[miroslavstastny]

This is needed as there are tests which are already copy&paste.

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