Add test for array.flat(undefined)

Author: sosukesuzuki

test/built-ins/Array/prototype/flat/non-numeric-depth-should-not-throw.js

@@ -41,3 +41,8 @@ depthNum = '1';
 actual = a.flat(depthNum);
 expected = [1, 2]
 assert.compareArray(actual, expected, 'The value of actual is expected to equal the value of expected');
+
+// undefined depthNum uses the default value of 1
+actual = a.flat(undefined);
+expected = [1, 2];
+assert.compareArray(actual, expected, 'a.flat(undefined) uses default depth of 1');