Update and improve the test for the length property of interface objects.
Update and improve the test for the length property of interface objects.

bboyle/testharness.js,jyasskin/testharness.js,zqzhang/testharness.js,jyasskin/testharness.js,hallvors/testharness.js,RatneshThakur/testharness.js,ayg/testharness.js,hallvors/testharness.js