Update TestKernel to match class loader changes in Drupal Kernel

Problem/Motivation

#2325197: Remove drupal_classloader() changed how the class loader works in DrupalKernel. This also needs to be done to TestKernel

Proposed resolution

Replicate the changes to DrupalKernel in TestKernel.

Remaining tasks

User interface changes

None.

API changes

None, though the type signature of TestKernel will remove the type hint for the $class_loader parameter.

Original report by @username

There is a type mismatch TestKernel and DrupalKernel for the createFromRequest method.

DrupalKernel:

public static function createFromRequest(Request $request, $class_loader, $environment, $allow_dumping = TRUE) { ... }

TestKernel:

public static function createFromRequest(Request $request, ClassLoader $class_loader, $environment, $allow_dumping = TRUE) { ... }

This causes problems when running some tests under PHP 5.5. For example, SimpleTestBrowserTest fails with PHP 5.5 IN HEAD. It passes when you add the proper type hint to DrupalKernel::createFromRequest().