tests: ignore xmlstarlet extra empty lines

With xmlstarlet 1.3.0 (which is on Ubuntu precise)

xmlstarlet sel -t -m '//pg_stat/state[(contains(.,'\''active'\'') and
contains(.,'\''clean'\'') and not(contains(.,'\''stale'\'')))]' -v . -n

will output an extra empty line which does not show on xmlstarlet
1.5.0 (which is on Ubuntu trusty and Fedora 20).

Signed-off-by: Loic Dachary <ldachary@redhat.com>
Merge pull request #3190 from dachary/wip-test-helpers

tests: ignore xmlstarlet extra empty lines