Provide error test assertions via a trait

Problem/Motivation

This issue came out in #2787873: Add a base class for entity reference selection handlers and fix the structure of their configuration.

As we are deprecating functions/methods we are starting to emit deprecation errors @trigger_error('...', E_USER_DEPRECATED). Right now there's now way to test such errors and, in general, all kind of errors that are not exceptions.

Proposed resolution

Provide a trait with two methods to be used with unit, kernel and browser/javascript tests:

trait ErrorTestTrait {
 
  protected function assertError($message, $severity) {...}

  protected function assertNoError($message, $severity) {...}

}

Remaining tasks

Discuss. Agree. Implement.

User interface changes

None.

API changes

New trait ^^^.

Data model changes

None.