Bug 12499: adding units tests for C4::Letters

Bug 12499: adding units tests for C4::Letters

The tests for the subroutines : GetMessageTransportType, EnqueueLetter, SendQueuedMessages, GetQueuedMessages, GetLetters, getletter, addalert, getalert, delalert and GetPreparedLetter of the module C4::Letters have been added.

Test plan:
1) Apply the patch
2) Execute the unit tests by launching :
prove t/db_dependent/Letters.t
3) The result has to be a success without error or warning :
t/db_dependent/Letters.t .. ok
All tests successful.
Files=1, Tests=45,  2 wallclock secs ( 0.04 usr  0.01 sys +  1.44 cusr  0.08 csys =  1.57 CPU)
Result: PASS

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
All test pass, no errors.

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>