mulh.S test has potential duplicates

[jsn1993]

https://github.com/riscv/riscv-tests/blob/master/isa/rv32um/mulh.S#L25-L26 Here these two tests are the same. While all other tests are identical to mulhsu/mulhu/mulh.S, the one on Line 26 isn't. (I doubt someone just copy-pasted it and didn't fix the second one)

[aswaterman]

Indeed, feel free to make a pull request that improves the test.