support for testing without /usr/share/doc/*/*

in order to catch maintainer scripts relying on /usr/share/doc content

keep the directories (or symlinks) and the copyright files to avoid
triggering other tests to fail

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p.conf: add [sid-nodoc] section

Signed-off-by: Andreas Beckmann <anbe@debian.org>