A general idea (applies to both conv and im2col). It makes sense to me to support multiple implementations of an op (specifically, here, one may be slower/naive but easier to verify correct or read and understand, while another may be more efficient).

Shall we adopt some naming convention for the methods in the classes that implement the ops to support this?

So, for example, your naive implementation of im2col can go back into the main class (instead of being in the testing code). We just use an appropriate suffix to distinguish the two implementations. Eg., may be add _optimized for the faster on. Or, add _spec to the naive implementation.