Fix test

Author: ondrejmirtes

tests/PHPStan/Analyser/Bug13813IntegrationTest.php

@@ -36,11 +36,11 @@ public function testBug13813(): void
 			);
 		} else {
 			$this->assertSame(
-				'Notice: Undefined variable $x',
+				'Notice: Undefined variable: x',
 				$analyzerResult->getAllPhpErrors()[0]->getMessage(),
 			);
 			$this->assertSame(
-				'Notice: Undefined variable $x',
+				'Notice: Undefined variable: x',
 				$analyzerResult->getAllPhpErrors()[1]->getMessage(),
 			);
 		}