tests: bypass the proxy if testing timeouts

If an explicit proxy is configured in the environment, the timeout tests
will actually go through it, and then the test results will depend on
the proxy's performance, which can vary.

Testing this is very simple:
```
export http_proxy="http://127.0.0.1:1234"
cargo test timeouts
```

We're hitting this issue in the Ubuntu CI. Further investigations
after 892569e seemed to point that
**sometimes**, the proxy would be too slow (particularly on non amd64
architectures), leading to the tests to actually timeout and pass.

tests/timeouts.rs

@@ -19,6 +19,7 @@ async fn client_timeout() {
 
     let client = reqwest::Client::builder()
         .timeout(Duration::from_millis(100))
+        .no_proxy()
         .build()
         .unwrap();
 
@@ -44,7 +45,7 @@ async fn request_timeout() {
         }
     });
 
-    let client = reqwest::Client::builder().build().unwrap();
+    let client = reqwest::Client::builder().no_proxy().build().unwrap();
 
     let url = format!("http://{}/slow", server.addr());
 
@@ -71,6 +72,7 @@ async fn connect_timeout() {
 
     let client = reqwest::Client::builder()
         .connect_timeout(Duration::from_millis(100))
+        .no_proxy()
         .build()
         .unwrap();
 
@@ -101,6 +103,7 @@ async fn connect_many_timeout_succeeds() {
             &["10.255.255.1:81".parse().unwrap(), server.addr()],
         )
         .connect_timeout(Duration::from_millis(100))
+        .no_proxy()
         .build()
         .unwrap();
 
@@ -128,6 +131,7 @@ async fn connect_many_timeout() {
             ],
         )
         .connect_timeout(Duration::from_millis(100))
+        .no_proxy()
         .build()
         .unwrap();
 
@@ -190,6 +194,7 @@ async fn read_timeout_applies_to_headers() {
 
     let client = reqwest::Client::builder()
         .read_timeout(Duration::from_millis(100))
+        .no_proxy()
         .build()
         .unwrap();
 
@@ -410,7 +415,7 @@ async fn response_body_timeout_forwards_size_hint() {
 
     let server = server::http(move |_req| async { http::Response::new(b"hello".to_vec().into()) });
 
-    let client = reqwest::Client::new();
+    let client = reqwest::Client::builder().no_proxy().build().unwrap();
 
     let url = format!("http://{}/slow", server.addr());