Remove remaining usage of setAccessible()

Usage is

core$ git grep setAccessible
core/tests/Drupal/Tests/Component/Plugin/Attribute/AttributeClassDiscoveryCachedTest.php:70:    $ref_file_cache->setAccessible(TRUE);
core/tests/Drupal/Tests/Component/Plugin/Attribute/AttributeClassDiscoveryCachedTest.php:128:    $ref_file_cache->setAccessible(TRUE);
core/tests/Drupal/Tests/Component/Plugin/Attribute/AttributeClassDiscoveryTest.php:51:    $reflection->setAccessible(TRUE);
core/tests/Drupal/Tests/Core/Entity/Access/EntityCreateAccessCustomCidTest.php:100:    $access_cache->setAccessible(TRUE);
core/tests/Drupal/Tests/Core/Field/FieldTypePluginManagerTest.php:75:    $property->setAccessible(TRUE);
core/tests/Drupal/Tests/Core/Theme/Icon/IconFinderTest.php:647:    $method->setAccessible(TRUE);
core/tests/Drupal/Tests/Core/Theme/Icon/IconFinderTest.php:661:    $method->setAccessible(TRUE);
core/tests/Drupal/Tests/Core/Theme/Icon/IconFinderTest.php:674:    $method->setAccessible(TRUE);
core/tests/Drupal/Tests/Core/Theme/Icon/IconFinderTest.php:688:    $method->setAccessible(TRUE);
core/tests/Drupal/Tests/Core/Theme/Icon/IconFinderTest.php:702:    $method->setAccessible(TRUE);
core/tests/Drupal/Tests/Core/Theme/Icon/IconFinderTest.php:768:    $method->setAccessible(TRUE);
core/tests/Drupal/Tests/Core/Theme/Icon/IconFinderTest.php:778:    $method->setAccessible(TRUE);

pretty straight forward, searched for setAccessible() and found 12 instances all covered by the MR.

Committed and pushed 967e3af639f to 11.x and d6d4d9cd564 to 11.2.x. Thanks!

andypost → closed merge request !12947