cesium: Erratic test failure in master

The last master build failed with the below error. If I had to guss, this test is using an external server. We should probably bring whatever data its using in-house to make sure CI doesn’t randomly fail.

CC @likangning93 @OmarShehata

  Core/GoogleEarthEnterpriseTerrainData
    upsample
      ✗ works for all four children of a simple quad
	Error: Timeout - Async callback was not invoked within 5000ms (set by jasmine.DEFAULT_TIMEOUT_INTERVAL) in node_modules/jasmine-core/lib/jasmine-core/jasmine.js (line 5096)
	<Jasmine>
5.002 secs: Core/GoogleEarthEnterpriseTerrainData upsample works for all four children of a simple quad
3.236 secs: Core/GoogleEarthEnterpriseTerrainData createMesh creates specified vertices plus skirt vertices
1.667 secs: Core/GoogleEarthEnterpriseMetadata resolves readyPromise
1.187 secs: Core/VideoSynchronizer Syncs time when not looping
1.151 secs: Core/ApproximateTerrainHeights initializes
1.132 secs: Scene/GroundPrimitive does not pick when allowPicking is false
1.109 secs: Scene/GroundPrimitive larger scene renders batched instances
1.087 secs: Scene/Vector3DTileContent renders polygons
1.069 secs: DataSources/GeometryVisualizer Creates and removes static color open geometry
1.053 secs: Scene/GroundPrimitive picking in 2D
Firefox 63.0.0 (Linux 0.0.0): Executed 10347 of 10355 (1 FAILED) (skipped 8) (2 mins 22.893 secs / 2 mins 13.272 secs)
TOTAL: 1 FAILED, 10346 SUCCESS