move tls type name to .cc file

test/cpp/util/test_credentials_provider.cc

@@ -39,6 +39,8 @@
 namespace grpc {
 namespace testing {
 
+const char kTlsCredentialsType[] = "TLS_CREDENTIALS";
+
 std::shared_ptr<ChannelCredentials> GetChannelCredentials(
     const grpc::string& type, ChannelArguments* args) {
   if (type == kInsecureCredentialsType) {

test/cpp/util/test_credentials_provider.h

@@ -44,7 +44,6 @@ namespace grpc {
 namespace testing {
 
 const char kInsecureCredentialsType[] = "INSECURE_CREDENTIALS";
-const char kTlsCredentialsType[] = "TLS_CREDENTIALS";
 
 // Provide channel credentials according to the given type. Alter the channel
 // arguments if needed.