tests: bypass the proxy if testing DNS override

If an explicit proxy is configured in the environment, then the request
will go through it rather than actually resolving the domain. Either
we're hitting the target domain on a weird port which will likely fail,
or the proxy straight up denies that weird request.

To work around this, we actually modify the environment variables to
make sure that our target domain is in the exception list for the proxy.

We're hitting this issue in the Ubuntu CI. Amazingly enough, the tests
actually passed *once* there, although the exact circumstances that
allowed this are still a bit of a mystery.

tests/client.rs

@@ -235,12 +235,31 @@ async fn body_pipe_response() {
     assert_eq!(res2.status(), reqwest::StatusCode::OK);
 }
 
+fn set_proxy_exception(domain: &str) {
+    let mut exception_set = false;
+    // We add it to *all* potential lists.
+    for name in ["NO_PROXY", "no_proxy"] {
+        if let Ok(v) = std::env::var(name) {
+            // Add the domain if not already present
+            if ! v.split(',').any(|dmn| {dmn == domain}) {
+                std::env::set_var(name, format!("{v},{domain}"));
+            }
+            exception_set = true;
+        }
+    }
+    // If nothing was set, we create the defaut list, just in case
+    if !exception_set {
+        std::env::set_var("NO_PROXY", domain);
+    }
+}
+
 #[tokio::test]
 async fn overridden_dns_resolution_with_gai() {
     let _ = env_logger::builder().is_test(true).try_init();
     let server = server::http(move |_req| async { http::Response::new("Hello".into()) });
 
     let overridden_domain = "rust-lang.org";
+    set_proxy_exception(overridden_domain);
     let url = format!(
         "http://{overridden_domain}:{}/domain_override",
         server.addr().port()
@@ -263,6 +282,7 @@ async fn overridden_dns_resolution_with_gai_multiple() {
     let server = server::http(move |_req| async { http::Response::new("Hello".into()) });
 
     let overridden_domain = "rust-lang.org";
+    set_proxy_exception(overridden_domain);
     let url = format!(
         "http://{overridden_domain}:{}/domain_override",
         server.addr().port()
@@ -297,6 +317,7 @@ async fn overridden_dns_resolution_with_hickory_dns() {
     let server = server::http(move |_req| async { http::Response::new("Hello".into()) });
 
     let overridden_domain = "rust-lang.org";
+    set_proxy_exception(overridden_domain);
     let url = format!(
         "http://{overridden_domain}:{}/domain_override",
         server.addr().port()
@@ -321,6 +342,7 @@ async fn overridden_dns_resolution_with_hickory_dns_multiple() {
     let server = server::http(move |_req| async { http::Response::new("Hello".into()) });
 
     let overridden_domain = "rust-lang.org";
+    set_proxy_exception(overridden_domain);
     let url = format!(
         "http://{overridden_domain}:{}/domain_override",
         server.addr().port()