add sample video

Summary:
add sample video. The video is about facebook video ads. We want to avoid any copyright issue
Video link: https://www.facebook.com/business/a/Facebook-video-ads?campaign_id=841106605909478&placement=exact&creative=54197424309&keyword=facebook+video+ads&extra_1=4c77585a-fe0d-5148-d383-0000561b6d7d

The video is the one on the top.

Test Plan: upload the video to my account

Author: Pingyang He

examples/test_video.mp4

@@ -28,7 +28,7 @@
     video = AdVideo(parent_id=my_account.get_id_assured())
 
     # set video fields
-    video[AdVideo.Field.filepath] = os.path.join(this_dir, 'test_video.m4v')
+    video[AdVideo.Field.filepath] = os.path.join(this_dir, 'test_video.mp4')
 
     # remove create
     video.remote_create()