nixos/tests/chromium: Split up into subtests

This makes it easier to test just a specific channel rather than to
force testing all builds down the users/testers throat. Especially this
makes it easier to test NixOS channel upgrades only against the Chromium
stable channel instead of just removing the beta/dev channels from the
tests entirely (as done in 69ec09f3

).

Signed-off-by: aszlig <aszlig@redmoonstudios.org>