Don’t run tests when installing CPAN modules

Go read this if you need a reason why:
http://www.openswartz.com/2012/01/31/stop-running-tests-on-install/
http://perldoc.perl.org/cpan.html
Ensure CPAN halts after the first failure

Normally, CPAN.pm continues processing the full list of targets and
dependencies, even if one of them fails. We would prefer our Omnibus
build terminates if there is trouble installing a Perl module’s dep!