Switch test_sky to sky_server

Adam did this in:
https://codereview.chromium.org/699333002
But later reverted in:
https://codereview.chromium.org/692333007

The problem was that the path to sky_server
was being computed relative to the sky
directory instead of being absolute, so when
the bots tried to run test_sky from the root
mojo directory instead of mojo/sky they failed.

TBR=abarth@chromium.org

Review URL: https://codereview.chromium.org/704833002