Use ObserveGroupOfPicture() in EncodeFrame test

Use ObserveGroupOfPicture() in EncodeFrame test

In the previous version, we assume the number of coding frames is
known.

Although the assumption is true for now with rate_ctrl flag on,
it's more proper to use ObserveGroupOfPicture() to get
the partial info about how many coding frames are in the group.

Because We want to keep the flexibility of changing the size of
group of pictures on the fly in the future.

Change-Id: Ibbe6ab49268c468bf1cef8344efd3a3e1eab972a