I understand the concern, but I value consistency and discoverability, so symmetry of thing-being-tested to test itself is (so far) the best way I've found to make sure it's dreadfully obvious where a given unit's test is.
This approach is not concerned with brittleness or being coupled to the implementation because each unit is so small that it's easier to trash the object and its test when requirements change than it is to try to update both dramatically.
This approach is not concerned with brittleness or being coupled to the implementation because each unit is so small that it's easier to trash the object and its test when requirements change than it is to try to update both dramatically.