Added assert methods for determining if text has been escaped properly

Two methods have been added to AssertContentTrait to test whether escaped text is present on a page (or not).

The following assertions

Drupal 7

$this->assertRaw(check_plan($string1));
$this->assertNoRaw(check_plain($string2));

Drupal 8 - before

$this->assertRaw(Html::escape($string1));
$this->assertNoRaw(Html::escape($string2));

Can be replaced with

$this->assertEscaped($string1);
$this->assertNoEscaped($string2);