AssertHelperTrait::castSafeStrings is no longer required

Previously, PHPunit was unable to directly compare string-like objects that implemented MarkupInterface, so AssertHelperTrait provided a castSafeStrings() method that could be used to convert these objects to string before comparing them. Drupal now provides a comparator class that can automatically convert and compare MarkupInterface objects, so the AssertHelperTrait and castSafeStrings() method have been deprecated; tests can now use assertEquals directly.

For example,

use AssertHelperTrait;

public function testSomething() {
  ...
  $this->assertSame($expected, $this->castSafeStrings($reasons));
}

can now be written as:

public function testSomething() {
  ...
  $this->assertEquals($expected, $reasons);
}