minor #21695 Refactored other PHPUnit method calls to work with namespaced PHPUnit 6 (peterrehm)

This PR was merged into the 2.8 branch.

Discussion
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Refactored other PHPUnit method calls to work with namespaced PHPUnit 6

| Q             | A
| ------------- | ---
| Branch?       | 2.8
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Continued work to make Symfony PHPUnit 6 compatible.

Commits
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dbe8898 Refactored other PHPUnit method calls to work with namespaced PHPUnit 6

Author: nicolas-grekas

Tests/DependencyInjection/Compiler/FormPassTest.php

@@ -227,7 +227,12 @@ public function testPrivateTaggedServices($id, $tagName, $expectedExceptionMessa
         $container->setDefinition('form.extension', $extDefinition);
         $container->register($id, 'stdClass')->setPublic(false)->addTag($tagName);
 
-        $this->setExpectedException('\InvalidArgumentException', $expectedExceptionMessage);
+        if (method_exists($this, 'expectException')) {
+            $this->expectException('InvalidArgumentException');
+            $this->expectExceptionMessage($expectedExceptionMessage);
+        } else {
+            $this->setExpectedException('InvalidArgumentException', $expectedExceptionMessage);
+        }
 
         $container->compile();
     }