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
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036a36cb14 Use ::class keyword when possible

Author: fabpot

Tests/Exception/FlattenExceptionTest.php

@@ -304,7 +304,7 @@ function () {},
 
         $args = $array[$i++];
         $this->assertSame('object', $args[0]);
-        $this->assertTrue('Closure' === $args[1] || is_subclass_of($args[1], '\Closure'), 'Expect object class name to be Closure or a subclass of Closure.');
+        $this->assertTrue('Closure' === $args[1] || is_subclass_of($args[1], \Closure::class), 'Expect object class name to be Closure or a subclass of Closure.');
 
         $this->assertSame(['array', [['integer', 1], ['integer', 2]]], $array[$i++]);
         $this->assertSame(['array', ['foo' => ['integer', 123]]], $array[$i++]);