Handle multiple async lifecycle functions in a suite

[jason0x43]

The hooks in the tdd and bdd interfaces use aspect.on to add lifecycle functions to suites, which is equivalent to aspect.after. If a hook is called multiple times in a suite, only the value from the last added hook will be returned and resolved by Intern.

describe('something', () => {
    beforeEach(() => {
        // return a promise
    });

    beforeEach(() => {
        // return a promise
    });
});

The interfaces should do something (like wrap the hook callback) to ensure that any promise returned by a previous call to the hook will be resolved before the current callback is called.