Fix parallel make

Make doesn't know that 'libmock_service.la' is the same as
'$(top_builddir)/library/tests/libmock_service.la', and fails to
properly order builds when using -jX with parallel make. Fix this by
using 'libmock_service.la' everywhere.

make[4]: *** No rule to make target `../../library/tests/libmock_service.la', needed by `test-value'.  Stop.