[Mono.CodeContracts] Always build debug file for the test assembly

The tests require that the debug file is available, however in the net_4_0 profile this file isn't generated by default.
We now ensure the debug file is always generated for the test assembly.
Merge pull request #1017 from akoeplinger/fix-monocc

[Mono.CodeContracts] Always build debug file for the test assembly