Most likely reason is that the function isn't listed in the export list of the DLL. Loading it into the dependency viewer could confirm that.
The solution would be a .def file fed to the linker.
It should also be mentioned that Test1 doesn't honor the __stdcall convention, which is the usual convention in DLLs. Neither does it honor the naming convention for __stdcall. And finally, it simply has no visible effect, because it increments a copy of the number passed. (I think. I'm not very familiar with MASM syntax.)