Fix test failure.

Author: kring

Source/Scene/Model.js

@@ -57,6 +57,7 @@ import Axis from './Axis.js';
 import BlendingState from './BlendingState.js';
 import ClippingPlaneCollection from './ClippingPlaneCollection.js';
 import ColorBlendMode from './ColorBlendMode.js';
+import DepthFunction from './DepthFunction.js';
 import DracoLoader from './DracoLoader.js';
 import getClipAndStyleCode from './getClipAndStyleCode.js';
 import getClippingFunction from './getClippingFunction.js';
@@ -3011,7 +3012,7 @@ import ShadowMode from './ShadowMode.js';
             },
             depthTest : {
                 enabled : true,
-                func: WebGLConstants.LEQUAL
+                func: DepthFunction.LESS_OR_EQUAL
             },
             depthMask : !blendingEnabled,
             blending : {

Specs/Scene/ModelSpec.js

@@ -32,6 +32,7 @@ import { DracoLoader } from '../../Source/Cesium.js';
 import { HeightReference } from '../../Source/Cesium.js';
 import { Model } from '../../Source/Cesium.js';
 import { ModelAnimationLoop } from '../../Source/Cesium.js';
+import { DepthFunction } from '../../Source/Cesium.js';
 import createScene from '../createScene.js';
 import pollToPromise from '../pollToPromise.js';
 import { when } from '../../Source/Cesium.js';
@@ -522,7 +523,8 @@ describe('Scene/Model', function() {
                     enabled : true
                 },
                 depthTest : {
-                    enabled : true
+                    enabled : true,
+                    func : DepthFunction.LESS_OR_EQUAL
                 },
                 depthMask : true,
                 blending : {