Avoid false positives when PHPUnit doesn't exist.

Author: codebymikey

src/Rules/Drupal/Tests/BrowserTestBaseDefaultThemeRule.php

@@ -17,6 +17,10 @@ public function getNodeType(): string
 
     public function processNode(Node $node, Scope $scope): array
     {
+        if (!interface_exists(\PHPUnit\Framework\Test::class)) {
+            // PHPUnit is unavailable in the class loader.
+            return [];
+        }
         assert($node instanceof Node\Stmt\Class_);
         if ($node->extends === null) {
             return [];