Use a separate toplevel target for venus/tests

There's no particular reason to lump them in with the venus target,
and we have reports that it causes parallel build failures on some
systems.

Just use a separate 'venustests' target akin to rxtests and ubiktests
and the like, instead.

Some of these lines are now long and should be wrapped, but leave
that for a follow-up commit.

Change-Id: Idd50c02d3c0c88dc2788ecfd221bbc5cbf8cba19
Reviewed-on: http://gerrit.openafs.org/11177
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>