std and race bench tests fail to check against t.runRxWant, so what
should be negative filters act as positive filters.

Fixes #19239

Change-Id: Icf02b2192bcd806a162fca9fb0af68a27ccfc936
Reviewed-on: https://go-review.googlesource.com/37336
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>