math/big: check return value from quick.Check() for GCD tests

Change-Id: I46c12aaaf453365c157604dfb1486605cfefd7af
Reviewed-on: https://go-review.googlesource.com/15263Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/math/big/int_test.go

@@ -695,7 +695,9 @@ 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{