Conformance tests for triggers

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

Start by locating the trigger-pattern components and their existing test setup. Add conformance coverage for consistent prop application, customer props taking precedence, and merged internal and user callbacks; done means the described markup and both callback behaviors are verified across applicable components.