runtime/debug: increase threshold on TestSetGCPercent
    
    Currently TestSetGCPercent checks that NextGC is within 10 MB of the
    expected value. For some reason it's much noisier on some of the
    builders. To get these passing again, raise the threshold to 20 MB.
    
    Change-Id: I14e64025660d782d81ff0421c1eb898f416e11fe
    Reviewed-on: https://go-review.googlesource.com/41374
    Run-TryBot: Austin Clements <austin@google.com>
    Reviewed-by: Russ Cox <rsc@golang.org>