minor #21918 [hhvm] 3.18-related fix (nicolas-grekas)

This PR was merged into the 3.2 branch.

Discussion
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[hhvm] 3.18-related fix

| Q             | A
| ------------- | ---
| Branch?       | 3.2
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

See #21914

Commits
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bc6c569 [hhvm] 3.18-related fix

src/Symfony/Component/HttpKernel/Tests/DependencyInjection/AddClassesToCachePassTest.php

@@ -21,6 +21,7 @@ class AddClassesToCachePassTest extends TestCase
         $r = new \ReflectionClass(AddClassesToCachePass::class);
         $pass = $r->newInstanceWithoutConstructor();
         $r = new \ReflectionMethod(AddClassesToCachePass::class, 'expandClasses');
+        $r->setAccessible(true);
         $expand = $r->getClosure($pass);
 
         $this->assertSame('Foo', $expand(array('Foo'), array())[0]);