Instead of requiring that the first call to setInterval() be setInterval(animate, 1000/30) just require that a call be made once.

setIntervalSpec.js

// ...snip.. from test.js:49

describe("setInterval", function() {
	it("should have been called", function() {
		setIntervalSpy.should.have.been.called;
	});

	it("with 'animate' as its first parameter", function() {
		setIntervalSpy.should.have.been.calledWith(animate);
	});

	it("with second parameter equal to a framerate of 30 frames per second", function() {
    var calls_to_animate_30_fps = 0;
    
    setIntervalSpy.args.forEach(function(args) {
      if  (args[0] === animate &&       // we called animate
          (args[1] - 1000/30) < 4 ) {   // we set the timeout to within four milliseconds of 30 FPS
          calls_to_animate_30_fps = calls_to_animate_30_fps + 1;
      }
    });
		expect(calls_to_animate_30_fps).to.be.equalTo(1, "We expected your framerate passed to setInterval to be within 4 frames per second of 30 frames per second.\nHint: setInterval takes its arguments in milliseconds, and there are 1000 milliseconds in a second.\n\n");
	});
});