Added function for parsing single rtcp packet in tests.
Added function for parsing single rtcp packet in tests.

BUG=webrtc:5260
R=pbos@webrtc.org

Review URL: https://codereview.webrtc.org/1810913002 .

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#12033}

ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc