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.

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>
(cherry picked from commit f66c467bdd7182eb80ede3b9faa5f5ae5231632f)

Change-Id: Ic0c1ae4d6adb265417ba726967ff02fed476da7e
Reviewed-on: http://gerrit.openafs.org/11282
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>