RenderCallbackRule: Check if method exists in trustedCallbacks

[Boegie]

Resolves #561

[mglaman]

😩

     Internal error: Internal error: Method of class                                                  
     Drupal\Tests\Core\Render\PlaceholdersTest cannot be used as the class                            
     does not exist in file                                                                           
     /home/runner/work/_temp/drupal/core/tests/Drupal/Tests/Core/Render/PlaceholderGeneratorTest.php  

That is because PlaceholdersTest is in core/tests/Drupal/Tests/Core/Render/RendererTestBase.php. The test class \Drupal\Tests\Core\Render\PlaceholderGeneratorTest extends \Drupal\Tests\Core\Render\RendererTestBase. But it's not brought into scope.

[mglaman]

We need to run PHPStan with phpstan-drupal and this PR against Drupal core using --debug, at least against RendererPlaceholdersTest to get the full stacktrace. I don't know why PHPStan can detect but PHP's internal reflection breaks.

[mglaman]

This didn't fix the internal error:

Error: Internal error: Internal error: Method of class Drupal\Tests\Core\Render\BubblingTest cannot be used as the class does not exist in file /home/runner/work/_temp/drupal/core/tests/Drupal/Tests/Core/Render/RendererBubblingTest.php

I don't know what to do.