[VideoAsArray] Simplify the code for __getitem__ and add more tests

2 jobs for dask-pipelines in 4 minutes and 37 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #219091
docker
build_linux_37

00:04:35

81.0%
passed #219090
macosx
build_macosx_37

00:04:32

81.0%