Extend the refresh tests.

Verify that a second refresh can happen after the first. This adds
coverage to ensure that a refresh flow keeps the refresh token intact by
showing that a second refresh can succeed.

tests/tests.rs

@@ -417,6 +417,28 @@ async fn test_refresh() {
     assert_eq!("accesstoken2", tok.as_str());
     _m.assert();
 
+    let _m = mockito::mock("POST", "/token")
+        .match_body(mockito::Matcher::Regex(
+            ".*client_id=9022167.*refresh_token=refreshtoken.*".to_string(),
+        ))
+        .with_body(
+            serde_json::json!({
+                "access_token": "accesstoken3",
+                "token_type": "Bearer",
+                "expires_in": 59,
+            })
+            .to_string(),
+        )
+        .expect(1)
+        .create();
+
+    let tok = auth
+        .token(&["https://googleapis.com/some/scope"])
+        .await
+        .expect("failed to get token");
+    assert_eq!("accesstoken3", tok.as_str());
+    _m.assert();
+
     let _m = mockito::mock("POST", "/token")
         .match_body(mockito::Matcher::Regex(
             ".*client_id=9022167.*refresh_token=refreshtoken.*".to_string(),