Skip proxy_test in race mode (#1584)

proxy_test.go

@@ -1,3 +1,5 @@
+// +build !race
+
 /*
  *
  * Copyright 2017 gRPC authors.

test/end2end_test.go

@@ -103,7 +103,7 @@ var (
 	})
 )
 
-var raceMode bool // set by race_test.go in race mode
+var raceMode bool // set by race.go in race mode
 
 type testServer struct {
 	security           string // indicate the authentication protocol used by this server.