net/http/httptest: protect against port reuse

Should make BSDs more reliable. (they seem to reuse ports
quicker than Linux)

Tested by hand with local modifications to force reuse on
Linux. (net/http tests failed before, pass now) Details in the
issue.

Fixes #4436

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/6847101

src/pkg/net/http/httptest/server.go

@@ -155,6 +155,10 @@ func NewTLSServer(handler http.Handler) *Server {
 func (s *Server) Close() {
 	s.Listener.Close()
 	s.wg.Wait()
+	s.CloseClientConnections()
+	if t, ok := http.DefaultTransport.(*http.Transport); ok {
+		t.CloseIdleConnections()
+	}
 }
 
 // CloseClientConnections closes any currently open HTTP connections