Bug 1708567: Avoid error when checking string_exists

We need to avoid the sprintf() error if lang string expects extra args.

We can do this by supplying function check_string rather than
format_string

behatnotneeded

Change-Id: Ibfc2d9a552f2d87b2f5f762bab2ff2ec76a73dca
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>
(cherry picked from commit 7f9b6945)