CI: error on dead/unused code for features (#146)

By using `RUSTFLAGS="-D dead_code -D unused-variables -D unused"`, we tell
the compiler to upgrade warnings for unused code to errors. This combined
with a more elaborate test matrix means we can have the CI catch feature
flag combos that cause warnings.

src/test/agent_test.rs

@@ -1,3 +1,5 @@
+#![allow(dead_code)]
+
 use crate::test;
 use crate::test::testserver::{read_headers, TestServer};
 use std::io::{self, Write};