L2E: Add more unit tests for GOP API

L2E: Add more unit tests for GOP API

Add unit tests for a 4 frame video, which could be considered as a
corner case.

Three different GOP settings are tested and verified as valid.
(1). The first GOP has 3 coding frames, no alt ref.
     The second GOP has 1 coding frame, no alt ref.
     The numer of coding frames is 4.
     Their frame types are: keyframe, inter_frame, inter_frame,
     golden_frame.

(2). The first GOP has 4 coding frames, use alt ref.
     The second GOP has 1 coding frame, which is the overlay of
     the first GOP's alt ref frame.
     The numer of coding frames is 5.
     Their types are: keyframe, alt_ref, inter_frame, inter_frame,
     overlay_frame.

(3). Only one GOP with 4 coding frames, do not use alt ref.
     The numer of coding frames is 4.
     Their types are: keyframe, inter_frame, inter_frame, inter_frame.

Change-Id: I4079ff5065da79834b363b1e1976f65efed3f91f