Make argument to step_func_done optional
Make argument to step_func_done optional

Useful if you're expecting an event or callback but don't want to assert stuff.
ayg/testharness.js,hallvors/testharness.js,RatneshThakur/testharness.js,zqzhang/testharness.js,jyasskin/testharness.js,bboyle/testharness.js,hallvors/testharness.js,jyasskin/testharness.js