fix(cts): Avoid generating tests with MXFP types

In negative tests, we chose the type to use for WrongInput tests from the full list of dtypes. That created a problem when a new dtype is added as we don't yet have all the support we need in the serialization library for it.

Work around it temporarily by ignoring dtypes with value above 12 (the current maximum). This should be removed once we merge the new types to the serialization library and we're ready to truly support them.

Also substitute the check for DType_MAX in dtype.h. This allows us to add new types to the serialization lib without causing problems as long as we don't actually use them.