VP9 SVC: Add tests for layer sync on base layer.

VP9 SVC: Add tests for layer sync on base layer.

Create tests for sync layer. The purpose of new tests is not to check
bitrate targeting, thus they're put in a new file.

Create a base class for svc tests, which is also inherited by svc datarate
tests, to reduce code redundancy.

Start decoding in the test from the frame of layer sync.

Change-Id: I7226d208279ad785873dffef51e0a8abef23b256