Factor out TestServer. (#98)

This creates a struct that encapsulates the test server setup, and adds
a shutdown method to clean up the thread when done.

src/test/mod.rs

@@ -14,6 +14,7 @@ mod query_string;
 mod range;
 mod redirect;
 mod simple;
+mod testserver;
 mod timeout;
 
 type RequestHandler = dyn Fn(&Unit) -> Result<Stream, Error> + Send + 'static;