factor::numeric::gcd: Add explicit test for the 0 case

src/uu/factor/src/numeric/gcd.rs

@@ -72,6 +72,10 @@ mod tests {
             gcd(1, a) == 1
         }
 
+        fn zero(a: u64) -> bool {
+            gcd(0, a) == a
+        }
+
         fn divisor(a: u64, b: u64) -> bool {
             // Test that gcd(a, b) divides a and b
             let g = gcd(a, b);