tests: add a shorter 720p test clip

niklas_1280_720_30.y4m 60 frames @ 30fps
only a small number of frames are being used; this reduces the test data
download size in non-perf-test cases by >500M.
retain niklas_1280_720_30.yuv for encode+decode perf tests

Change-Id: I56b3433104acd462f952a9554280de5a3ec0b6d2