net/http: make TestUseProxy repeatable

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

src/pkg/net/http/proxy_test.go

@@ -35,12 +35,8 @@ var UseProxyTests = []struct {
 }
 
 func TestUseProxy(t *testing.T) {
-	oldenv := os.Getenv("NO_PROXY")
-	defer os.Setenv("NO_PROXY", oldenv)
-
-	no_proxy := "foobar.com, .barbaz.net"
-	os.Setenv("NO_PROXY", no_proxy)
-
+	ResetProxyEnv()
+	os.Setenv("NO_PROXY", "foobar.com, .barbaz.net")
 	for _, test := range UseProxyTests {
 		if useProxy(test.host+":80") != test.match {
 			t.Errorf("useProxy(%v) = %v, want %v", test.host, !test.match, test.match)
@@ -76,3 +72,10 @@ func TestCacheKeys(t *testing.T) {
 		}
 	}
 }
+
+func ResetProxyEnv() {
+	for _, v := range []string{"HTTP_PROXY", "http_proxy", "NO_PROXY", "no_proxy"} {
+		os.Setenv(v, "")
+	}
+	ResetCachedEnvironment()
+}

src/pkg/net/http/transport_test.go

@@ -1694,10 +1694,7 @@ var proxyFromEnvTests = []proxyFromEnvTest{
 }
 
 func TestProxyFromEnvironment(t *testing.T) {
-	os.Setenv("HTTP_PROXY", "")
-	os.Setenv("http_proxy", "")
-	os.Setenv("NO_PROXY", "")
-	os.Setenv("no_proxy", "")
+	ResetProxyEnv()
 	for _, tt := range proxyFromEnvTests {
 		os.Setenv("HTTP_PROXY", tt.env)
 		os.Setenv("NO_PROXY", tt.noenv)