Use a testserver in tests. (#194)

This is a step towards allowing our tests to run without network access,
which will make them more resilient and faster.

Replace the URL in one instance of an HTTPS test that didn't need HTTPS.

src/agent.rs

@@ -387,7 +387,7 @@ mod tests {
         use std::io::Read;
 
         let agent = crate::agent();
-        let url = "https://ureq.s3.eu-central-1.amazonaws.com/sherlock.txt";
+        let url = "http://example.com";
         // req 1
         let resp = agent.get(url).call().unwrap();
         let mut reader = resp.into_reader();