This is the double CR/LF. The client.println is one, and the "\r\n" is the other.
client.println("Content-Length: 9\r\n");
This string is nine characters long:
client.print("testone=1");
All after the double CR/LF is the content.
edit: I could have sent the double CR/LF like this if it makes it clearer for you
client.print("Content-Length: 9\r\n\r\n");
CR/LF = "Carriage Return/Line Feed"