Add Atest tab completion script.

Now Atest can autocomplete module names, dir and filenames with
tab completion, and zsh users can benefit from it.

Bug: 110629292
Test: In both bash and zsh environments:
    source build/envsetup.sh && lunch aosp_arm-eng
    atest <tab><tab>  # has candidates
    adb <tab><tab>    # has candidates
    ENVSETUP_NO_COMPLETION=atest:adb . build/envsetup.sh && lunch aosp_arm-eng
    atest <tab><tab>  # no candidates
    adb <tab><tab>    # no candidates

Change-Id: Ib1c9e02feeb8aaf75c0b97821ae26e13ba8df350