runtime/pprof: run TestGoroutineSwitch for longer
runtime/pprof: run TestGoroutineSwitch for longer

Short test now takes about 0.5 second here.

Fixes issue 6417.

The failure was also seen on our builders.

R=golang-dev, minux.ma, r
CC=golang-dev
https://codereview.appspot.com/13321048

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,tv42/old-go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme