minor #39775 [WIP] Use ::class keyword when possible (fabpot)

This PR was merged into the 4.4 branch.

Discussion
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[WIP] Use ::class keyword when possible

| Q             | A
| ------------- | ---
| Branch?       | 4.4 <!-- see below -->
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tickets       | n/a  <!-- prefix each issue number with "Fix #", no need to create an issue if none exist, explain below instead -->
| License       | MIT
| Doc PR        | n/a

Commits
-------

036a36cb14 Use ::class keyword when possible

Author: fabpot

Tests/Security/User/EntityUserProviderTest.php

@@ -21,6 +21,7 @@
 use Symfony\Bridge\Doctrine\Test\DoctrineTestHelper;
 use Symfony\Bridge\Doctrine\Tests\Fixtures\User;
 use Symfony\Component\Security\Core\User\PasswordUpgraderInterface;
+use Symfony\Component\Security\Core\User\UserInterface;
 
 class EntityUserProviderTest extends TestCase
 {
@@ -154,7 +155,7 @@ public function testLoadUserByUserNameShouldLoadUserWhenProperInterfaceProvided(
             ->method('loadUserByUsername')
             ->with('name')
             ->willReturn(
-                $this->getMockBuilder('\Symfony\Component\Security\Core\User\UserInterface')->getMock()
+                $this->getMockBuilder(UserInterface::class)->getMock()
             );
 
         $provider = new EntityUserProvider(