Return from tests that require extensions

The following tests are skipped:
* Scene/Primitive renders with distance display condition per instance attribute (floatingPointTextures)
* Scene/GroundPrimitive renders with distance display condition per instance attribute (floatingPointTextures)
* Scene/BillboardCollection renders more than 16k billboards (instancedArray)

Specs/Scene/BillboardCollectionSpec.js

@@ -917,6 +917,10 @@ defineSuite([
     });
 
     it('renders more than 16K billboards', function() {
+        if(!context.instancedArrays) {
+            return;
+        }
+
         for ( var i = 0; i < 16 * 1024; ++i) {
             billboards.add({
                 position : Cartesian3.ZERO,

Specs/Scene/GroundPrimitiveSpec.js

@@ -493,6 +493,10 @@ defineSuite([
     });
 
     it('renders with distance display condition per instance attribute', function() {
+        if (!context.floatingPointTexture) {
+            return;
+        }
+
         if (!GroundPrimitive.isSupported(scene)) {
             return;
         }

Specs/Scene/PrimitiveSpec.js

@@ -723,6 +723,10 @@ defineSuite([
     });
 
     it('renders with distance display condition per instance attribute', function() {
+        if (!context.floatingPointTexture) {
+            return;
+        }
+
         var near = 10000.0;
         var far = 1000000.0;
         var rect = Rectangle.fromDegrees(-1.0, -1.0, 1.0, 1.0);