Currently it fails on second and subsequent runs (when using -cpu=1,2,4) as:
--- FAIL: TestUseProxy-4 (0.00 seconds)
proxy_test.go:109: useProxy(barbaz.net) = true, want false
proxy_test.go:109: useProxy(foobar.com) = true, want false
proxy_test.go:109: useProxy(www.foobar.com) = true, want false

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/71940044