Add a test for the sector app.

This also moves the test for stocks into a new tests/examples/
directory, where we can put tests that test the examples.

TBR=abarth

Review URL: https://codereview.chromium.org/1182223004.

examples/widgets/sector.dart

@@ -28,7 +28,7 @@ RenderBox initCircle() {
 class SectorApp extends App {
 
   RenderBoxToRenderSectorAdapter sectors = initCircle();
-  math.Random rand = new math.Random();
+  math.Random rand = new math.Random(1);
 
   void addSector() {
     double deltaTheta;