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

This PR was merged into the 4.4 branch.

Discussion
----------

[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/ParserTest.php

@@ -68,7 +68,7 @@ public function testTabsInYaml()
 
                 $this->fail('YAML files must not contain tabs');
             } catch (\Exception $e) {
-                $this->assertInstanceOf('\Exception', $e, 'YAML files must not contain tabs');
+                $this->assertInstanceOf(\Exception::class, $e, 'YAML files must not contain tabs');
                 $this->assertEquals('A YAML file cannot contain tabs as indentation at line 2 (near "'.strpbrk($yaml, "\t").'").', $e->getMessage(), 'YAML files must not contain tabs');
             }
         }
@@ -1415,7 +1415,7 @@ public function testParseDateAsMappingValue()
      */
     public function testParserThrowsExceptionWithCorrectLineNumber($lineNumber, $yaml)
     {
-        $this->expectException('\Symfony\Component\Yaml\Exception\ParseException');
+        $this->expectException(ParseException::class);
         $this->expectExceptionMessage(sprintf('Unexpected characters near "," at line %d (near "bar: "123",").', $lineNumber));
 
         $this->parser->parse($yaml);