math/big: flip long/short flag on TestFloat32Distribution

It looks like a typo in CL 30707.

Change-Id: Ia2d013567dbd1a49901d9be0cd2d5a103e6e38cf
Reviewed-on: https://go-review.googlesource.com/32187
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/math/big/rat_test.go

@@ -383,7 +383,7 @@ func TestFloat32Distribution(t *testing.T) {
 		11,
 	}
 	var winc, einc = uint64(5), 15 // quick test (~60ms on x86-64)
-	if testing.Short() {
+	if *long {
 		winc, einc = uint64(1), 1 // soak test (~1.5s on x86-64)
 	}