Bug 12753: Warnings in t/SIP_Sip.t could be tested

Bug 12753: Warnings in t/SIP_Sip.t could be tested

A C4::SIP warning is expected and should be tested.

To test:
- Run
  $ prove -v t/SIP_Sip.t
=> FAIL: A warning is printed to STDOUT
- Apply the patch
- Run
  $ prove -v t/SIP_Sip.t
=> SUCCESS: No warnings on STDOUT, a new test verifying the warning is added.
- It was an easy one, smile and sign off :-D

Sponsored-by: The Will to Procrastinate
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Sponsored-by: Sunday TV is boring
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Sponsored-by: Avoiding distance study homework
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>