Remove duplicate building of mock_dcp.cc

mock_dcp.cc (which is used by a number of tests as a mock DCP implementation) is compiled multiple times as part of the build: ep_testsuite, ep_testsuite_basic, ep_perfsuite etc all build a copy of it. Similary for ep_test_apis.cc (and maybe others...?) - see kv_engine/engines/ep/CMakeLists.txt).

This results in unnecessary compilation (and increases compile-times). We should just compile these files once; and have each target which requires them link to a common object library.