Reduce the number of threads iron uses during tests

It defaults to 8 * num_cpus, and we leak these threads since we can't
shut iron down, so on my 20-core machine it was hitting 11,500 idle
threads during the test run.

Author: Nemo157

src/web/mod.rs

@@ -430,7 +430,11 @@ impl Server {
         context: &dyn Context,
     ) -> Result<Self, Error> {
         let cratesfyi = CratesfyiHandler::new(template_data, context)?;
-        let inner = Iron::new(cratesfyi)
+        let mut iron = Iron::new(cratesfyi);
+        if cfg!(test) {
+            iron.threads = 1;
+        }
+        let inner = iron
             .http(addr)
             .unwrap_or_else(|_| panic!("Failed to bind to socket on {}", addr));