the int conversion i think is unnessesary. in go division returns the same type and in integer case it returns the quotient

As has already been mentioned, the written instructions should be updated so it's clearer as to what is expected or what kind of tests it will be run against.
It is not stated that the variables may be negative, or n may be > m, and need to be checked for validity (E.g. n,m > 0 && n < m).

I agree with Markus.
I would recommend adding a sample test with n=25 and m=200. This will result in a condition where m is exactly equal to a multiple of n. Without it, the final test will sometimes pass and sometimes fail because the random test does not always include a scenario where m is an exact multiple of n.

Assert.AreEqual(700, Kata.SumMul(25, 200));

I think that change will eliminate the confusion people are encountering.

The description is misleading: positivity is not an assumption, it must me validated for by your code.