minor PHP-CS-Fixer#4471 ReadmeCommandTest - use CommandTester (kubawerlos)

This PR was merged into the 2.12 branch.

Discussion
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ReadmeCommandTest - use CommandTester

Fixes [Travis](https://travis-ci.org/FriendsOfPHP/PHP-CS-Fixer/jobs/561270967#L966) on PHP `7.4snapshot` by modernizing `ReadmeCommand` test.

Commits
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dbe4ed1 Test ReadmeCommand with CommandTester

Author: SpacePossum

tests/Console/Command/ReadmeCommandTest.php

@@ -14,9 +14,7 @@
 
 use PhpCsFixer\Console\Application;
 use PhpCsFixer\Tests\TestCase;
-use Symfony\Component\Console\Input\ArrayInput;
-use Symfony\Component\Console\Output\BufferedOutput;
-use Symfony\Component\Console\Output\OutputInterface;
+use Symfony\Component\Console\Tester\CommandTester;
 
 /**
  * @author Dariusz Rumiński <dariusz.ruminski@gmail.com>
@@ -36,15 +34,12 @@ public function testIfReadmeFileIsCorrect()
         $fileContent = file_get_contents($readmeFile);
         static::assertInternalType('string', $fileContent, sprintf('Failed to get content of "%s"', $readmeFile));
 
-        $app = new Application();
-        $input = new ArrayInput(['readme']);
+        $application = new Application();
 
-        $output = new BufferedOutput();
-        $output->setVerbosity(OutputInterface::VERBOSITY_VERY_VERBOSE);
-        $output->setDecorated(false);
+        $commandTester = new CommandTester($application->get('readme'));
 
-        $exitCode = $app->get('readme')->run($input, $output);
-        $output = $output->fetch();
+        $exitCode = $commandTester->execute([]);
+        $output = $commandTester->getDisplay();
         // normalize line breaks, these are not important for the tests
         $output = str_replace(PHP_EOL, "\n", $output);