tests: use ^ instead of ! as invalid char

Because !!! is incorrectly interpreted as an error by gitbuilder.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #1511 from dachary/wip-exclamation

tests: use ^ instead of ! as invalid char

Reviewed-by: Sage Weil <sage@inktank.com>