math/big: test that subVW and addVW work with arbitrary y

Fixes #10525.

Change-Id: I92dc87f5d6db396d8dde2220fc37b7093b772d81
Reviewed-on: https://go-review.googlesource.com/9210Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/big/arith.go

@@ -196,7 +196,6 @@ func subVV_g(z, x, y []Word) (c Word) {
 	return
 }
 
-// Argument y must be either 0 or 1.
 // The resulting carry c is either 0 or 1.
 func addVW_g(z, x []Word, y Word) (c Word) {
 	if use_addWW_g {

src/math/big/arith_test.go

@@ -155,6 +155,7 @@ var sumVW = []argVW{
 	{nat{1}, nat{1}, 0, 0},
 	{nat{0}, nat{_M}, 1, 1},
 	{nat{0, 0, 0, 0}, nat{_M, _M, _M, _M}, 1, 1},
+	{nat{585}, nat{314}, 271, 0},
 }
 
 var prodVW = []argVW{