Add integration tests for transitions

[lucaswoj]

Migrated from mapbox/mapbox-gl-test-suite#168

Per mapbox/mapbox-gl-style-spec#171, let's add tests for transition properties. Sticking point is somehow taking a screenshot after a certain elapsed time: @lucaswoj pointed me to a wait operation that waits til rendering is complete, but it doesn't support a duration argument yet.

• extend wait to include a duration parameter in gl-js
• add tests with durations to test transitions

Migrated from mapbox/mapbox-gl-test-suite#116

We should to figure out how to add tests for property transitions, in order to prevent bugs like:

• Text translate is throwing an error #2762
• Fix transitions from property default values #2769

A prerequisite for this is that the test harness has control over the clock used for rendering transitions, so it can deterministically render, say, 50% of the way through a transition. Then we could add that percentage as a parameter somewhere in the test case metadata.

gl-js internals which use Date.now() directly that need to be converted to using a passed in parameter:

mapbox-gl-js/js/style/style_declaration.js

Line 53 in 062f52d

var t = Math.min((Date.now() - zh.lastIntegerZoomTime) / duration, 1);

mapbox-gl-js/js/style/style_transition.js

Line 54 in 8eaeebb

var t = globalProperties.time || Date.now();

mapbox-gl-js/js/style/style_layer.js

Line 311 in 8eaeebb

newTransition.loopID = animationLoop.set(newTransition.endTime - Date.now());

mapbox-gl-js/js/render/frame_history.js

Line 17 in 1df1466

var now = Date.now();

mapbox-gl-js/js/style/style.js

Lines 250 to 257 in 7840d9c

	if (Math.floor(zh.lastZoom) < Math.floor(z)) {
	zh.lastIntegerZoom = Math.floor(z);
	zh.lastIntegerZoomTime = Date.now();
	} else if (Math.floor(zh.lastZoom) > Math.floor(z)) {
	zh.lastIntegerZoom = Math.floor(z + 1);
	zh.lastIntegerZoomTime = Date.now();
	}

Migrator's note: sinon has some built-in utilities for mocking out the system clock http://sinonjs.org/docs/#clock

[anandthakker]

Another issue to add test for once we've made it possible in the harness: #2467