Create proper test method for determining if text has been escaped properly

Currently, 8.0.x HEAD has seven cases where there are tests to determine if text has been escaped properly. The tests look similar to

$this->assertRaw(String::checkPlain($some_string));

While this works, it isn't the best idea. For one, it forces the developer to program to the implementation rather than the interface. It also violates a testing principle where the assertion should describe what you are testing, and not just generate a pass/fail.

AssertContentTrait should have a method for testing whether a string has been escaped or not in HTML output. Something like

  protected function assertEscaped($raw, $message = '', $group = 'Other') {
    if (!$message) {
      $message = String::format('Escaped "@raw" found', array('@raw' => $raw));
    }
    return $this->assert(strpos($this->getRawContent(), String::checkPlain($raw)) !== FALSE, $message, $group);
  }

should be sufficient.

I'll provide a patch if anyone thinks this is a good idea.