refactor: extract shared test infrastructure into testutil module

- test_ctx(): single ServerCtx builder, replaces 3 copies (ctx/api/dot)
- mock_upstream(): canned DNS response server for forwarding tests
- blackhole_upstream(): unresponsive socket for timeout tests
- Removes ~100 lines of duplicated 30-field struct literals

src/lib.rs

@@ -28,6 +28,9 @@ pub mod system_dns;
 pub mod tls;
 pub mod wire;
 
+#[cfg(test)]
+pub(crate) mod testutil;
+
 pub type Error = Box<dyn std::error::Error + Send + Sync>;
 pub type Result<T> = std::result::Result<T, Error>;