Unit test project in CMake with dependency, without transitive dependencies?

I'm working on a big project and we're having trouble properly using mocks in our unit tests. The main problem is that we haven't got all dependencies set correctly, but in solving this we find that because of transitive dependencies the unit test projects get a link to the implementation rather than to their mock.

I think this would be solved by making it depend on its unit-under-test without its dependencies, then the mocks, and then any explicit thing you'd want to add to that, but I can't find out how to depend only on the unit-under-test rather than it plus dependencies.

How can I fix this?