Fix unit tests on Nodejs prior 16

Author: lahmatiy

test/utils/tester.js

@@ -1,9 +1,10 @@
 // Reworked stylelint-rule-tester
 
 import assert, { deepStrictEqual } from 'assert';
-import { isRegExp } from 'util/types';
 import postcss from 'postcss';
 
+const isRegExp = value => toString.call(value) === '[object RegExp]';
+
 /**
  * Create a ruleTester for a specified rule.
  *