cmd/compile/internal/big: fix unused result from testing/quick.Check

Update #12834.

Change-Id: If7bbcc249517f2f2d8a7dcbba6411ede92331abe
Reviewed-on: https://go-review.googlesource.com/15381Reviewed-by: Damian Gryski <dgryski@gmail.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/big/int_test.go

@@ -698,7 +698,9 @@ func TestGcd(t *testing.T) {
 		testGcd(t, d, x, y, a, b)
 	}
 
-	quick.Check(checkGcd, nil)
+	if err := quick.Check(checkGcd, nil); err != nil {
+		t.Error(err)
+	}
 }
 
 var primes = []string{