zdtm.py: fix checkskip logic
zdtm.py: fix checkskip logic

Fix checkskip logic, also emit a clearer reason when checkskip fails as to
why the test was skipped.

Signed-off-by: Tycho Andersen <10c18fcb898ee6b17d6575d444b125d10b3f4d6a@canonical.com>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@virtuozzo.com>

AuthenticEshkinKot/criu,ldu4/criu,efiop/criu,KKoukiou/criu-remote,ldu4/criu,AuthenticEshkinKot/criu,eabatalov/criu,eabatalov/criu,AuthenticEshkinKot/criu,efiop/criu,eabatalov/criu,eabatalov/criu,efiop/criu,KKoukiou/criu-remote,KKoukiou/criu-remote,efiop/criu,eabatalov/criu,tych0/criu,AuthenticEshkinKot/criu,efiop/criu,eabatalov/criu,tych0/criu,ldu4/criu,tych0/criu,AuthenticEshkinKot/criu,ldu4/criu,tych0/criu,KKoukiou/criu-remote,tych0/criu,KKoukiou/criu-remote,tych0/criu,efiop/criu,ldu4/criu,ldu4/criu,KKoukiou/criu-remote,AuthenticEshkinKot/criu