Fix warn-on-top-level-this test

test/function/warn-on-top-level-this/_config.js

@@ -3,9 +3,10 @@ const assert = require( 'assert' );
 module.exports = {
 	description: 'warns on top-level this (#770)',
 	warnings: warnings => {
-		assert.deepEqual( warnings, [
-			'The `this` keyword is equivalent to `undefined` at the top level of an ES module, and has been rewritten'
-		]);
+		const message = `The 'this' keyword is equivalent to 'undefined' at the top level of an ES module, and has been rewritten`;
+		assert.equal(warnings.length, 1);
+		assert.equal(warnings[0].indexOf(message), 0);
+		assert(warnings[0].match(/\(in.*warn-on-top-level-this.*\)/));
 	},
 	runtimeError: err => {
 		assert.equal( err.message, `Cannot set property 'foo' of undefined` );