Looking at the CI logs, the p4 and cvs tests account for another 24
minutes of test time and they offer minimal value for quite a
similar reason as the previous step.
Let's introduce and use a mechanism to skip these tests to save
some resources.
Suggested-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>