Change default for consistency

Author: sebastianbergmann

src/TextUI/ShellExitCodeCalculator.php

@@ -27,7 +27,7 @@ public function calculate(Configuration $configuration, TestResult $result): int
     {
         $failOnDeprecation        = false;
         $failOnPhpunitDeprecation = false;
-        $failOnPhpunitWarning     = false;
+        $failOnPhpunitWarning     = true;
         $failOnEmptyTestSuite     = false;
         $failOnIncomplete         = false;
         $failOnNotice             = false;