SurfaceTexture: fix a couple tests

This change fixes some robustness issues with the
EglDestroySurfaceUnrefsBuffers and
EglDestroySurfaceAfterAbandonUnrefsBuffers tests.  The tests previously
depended upon GL implementation details that should not have been relied
upon.

Change-Id: I37fd43b56568efe1dbe69d85e892be8a1cf44d20