[RISCV] Add tests for vmadd for VP intrinsics. NFC (#70042)

We have VP tests for vmacc but not vmadd. This copies the vmacc tests
but swaps
the false operand of vp.merge to be the multiplicand instead of the
addend.

This shows how we could fold the vmerge into the vmadd's mask if we
commuted %a
and %b.