Bug 14111: Silence warnings in t/Auth_with_shibboleth.t

Bug 14111: Silence warnings in t/Auth_with_shibboleth.t

Poorly capturing warnings_exist test cases generated noise.
This patch fixes that.

TEST PLAN
---------
1) $ prove t/Auth_with_shibboleth.t
   -- There are messages about lines 132 and 133. Noise.
2) Apply patch
3) $ prove t/Auth_with_shibboleth.t
   -- No messages. YAY!
4) koha qa test tools

Signed-off-by: Indranil Das Gupta (L2C2 Technologies) <indradg@gmail.com>
Test remarks: Expected outcome as per test plan. No anomaly noted.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>