Test animation with missing from and to keyframes

This test performs animations of the left property on four boxes over 2 seconds. Box 1 has all keyframes. Box 2 has a missing "from" keyframe. Box 3 has a missing "to" keyframe. Box 4 has both "from" and "to" keyframes missing, but other keyframes which should trigger the generation of "from" and "to". The test takes 3 snapshots each and expects each result to be within a specified range.