tests: add missing libcore dependency after 75ee2f8c

$ (cmake -DENABLE_SDL2:BOOL=false /path/to/citra; gmake)
[...]
[ 85%] Linking CXX executable tests
../common/libcommon.a(microprofile.cpp.o): In function `MicroProfileThreadStart(pthread**, void* (*)(void*))':
src/common/microprofile.cpp:(.text+0x41): undefined reference to `pthread_create'
c++: error: linker command failed with exit code 1 (use -v to see invocation)