Canvas2D test enhancement

engine:
   1. remove the additional code that worked only for canvas fps tests.
   2. format the result string with smaller numbers of fraction digits.

canvas:
  1. fix the issue that may return 0 fps when the browser is quite slow
  2. reports an interval for 50 frames (the same fromat with other test cases)
  3. refact canvas test, make the code easier to be read