`stopTimer` tests aren't deterministic

[anderoonies]

Describe the bug
Tests for stopTimer assume a race condition, which almost always works, but occasionally fail. The test currently uses requestAnimationFrame to try to put itself at the back of the event loop, but there may be a better way to do it.

This is the relevant code, in graphics.test.js:

describe('Stopping a timer', () => {
    it('Can be stopped by identity', () => {
        const g = new Graphics();
        const timerFn = jasmine.createSpy();
        g.setTimer(timerFn, 0);
        g.stopTimer(timerFn);
        return new Promise(resolve => {
            requestAnimationFrame(() => {
                expect(timerFn).not.toHaveBeenCalled();
                resolve();
            });
        });
    });
    it('Can be stopped by name', () => {
        const g = new Graphics();
        const spy = jasmine.createSpy();
        g.setTimer(spy, 0);
        return new Promise(resolve => {
            requestAnimationFrame(() => {
                expect(spy).toHaveBeenCalled();
                // all jasmine spies are named 'wrap'
                g.stopTimer('wrap');
                requestAnimationFrame(() => {
                    expect(spy).toHaveBeenCalledTimes(1);
                    resolve();
                });
            });
        });
    });
    it('stopAllTimers stops all timers', () => {
        const g = new Graphics();
        const timerOne = jasmine.createSpy();
        const timerTwo = jasmine.createSpy();
        g.setTimer(timerOne, 0, {}, 'one');
        g.setTimer(timerTwo, 0, {}, 'two');
        g.stopAllTimers();
        return new Promise((resolve, reject) => {
            requestAnimationFrame(() => {
                expect(timerOne).not.toHaveBeenCalled();
                expect(timerTwo).not.toHaveBeenCalled();
                resolve();
            });
        });
    });
    });