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/test/redirect.rs

@@ -85,6 +85,9 @@ fn redirect_get() {
 #[test]
 fn redirect_host() {
     // Set up a redirect to a host that doesn't exist; it should fail.
+    // TODO: This actually relies on the network for the DNS lookup
+    // of example.invalid. We can probably do better by, e.g.
+    // overriding the resolver.
     let srv = TestServer::new(|mut stream: TcpStream| -> io::Result<()> {
         testserver::read_headers(&stream);
         write!(stream, "HTTP/1.1 302 Found\r\n")?;
@@ -93,8 +96,13 @@ fn redirect_host() {
         Ok(())
     });
     let url = format!("http://localhost:{}/", srv.port);
-    let resp = crate::get(&url).call();
-    assert!(matches!(resp.err(), Some(Error::DnsFailed(_))));
+    let resp = crate::Agent::default().get(&url).call();
+    let err = resp.err();
+    assert!(
+        matches!(err, Some(Error::DnsFailed(_))),
+        "expected DnsFailed, got: {:?}",
+        err
+    );
 }
 
 #[test]