tests: fix dejagnu pwdx and vmstat checks

tests: fix dejagnu pwdx and vmstat checks

The pwdx check expected error message, which where in use for only
short period of time.  The old message was put in place in commit
9d47cb0c38137581a46951fb7c6fa2a1dc8c24cf

The vmstat issue was triggered by 'sr0' cdrom device, which gives
following unexpected output.  Fix simply ignores partitions which has
zero activity.  Besides such partitions probably would not apply as
good candidate of -p option anyway.

$ vmstat -p sr0
Partition was not found

Reported-By: Moritz Muehlenhoff <jmm@debian.org>
Bug-Debian: http://bugs.debian.org/656508
Signed-off-by: Sami Kerola <kerolasa@iki.fi>