Create a TestConfig struct to separate setup

I was about to copy-paste the entire body of the attach_file_exists test
function into the attach_file_missing function. The only difference is
the file that they upload -- or don't, in the second case. I could make
a try-file-upload function and pass it many different files, but I don't
think I need that. Instead, I'll separate the test setup from the test
sequence itself.

src/api/release_attachment.rs

@@ -50,6 +50,43 @@ mod tests {
 
     #[tokio::test]
     async fn attach_file_exists() {
+        let conf = TestConfig::new();
+        let release_candidates =
+            crate::api::release::list_releases(
+                &conf.client,
+                &conf.server,
+                &conf.repo
+            )
+            .await
+            .expect("Failed to get releases. Pre-conditions unmet, aborting test!");
+
+        let release = match_release_by_tag(&conf.release_tag, release_candidates)
+            .expect("Failed to select matching release. Pre-conditions unmet, aborting test!");
+        
+        let api_result = super::create_release_attachment(
+            &conf.client,
+            &conf.server,
+            &conf.repo,
+            release.id,
+            vec![String::from("Cargo.toml")],
+        )
+        .await;
+    }
+
+    #[test]
+    fn attach_file_missing() {
+        todo!();
+    }
+
+    struct TestConfig {
+        server: String,
+        repo: String,
+        release_tag: String,
+        client: reqwest::Client,
+    }
+
+    impl TestConfig {
+        fn new() -> Self {
             let server = std::env::var("TEST_GITEA_SERVER")
                 .expect("Must set server address in env var \"TEST_GITEA_SERVER\"");
             let repo = std::env::var("TEST_GITEA_REPO")
@@ -75,31 +112,13 @@ mod tests {
                 .build()
                 .expect("Failed to build reqwest::Client.");
 
-        let release_candidates =
-            crate::api::release::list_releases(
-                &client,
-                &server,
-                &repo
-            )
-            .await
-            .expect("Failed to get releases. Pre-conditions unmet, aborting test!");
-
-        let release = match_release_by_tag(&release_tag, release_candidates)
-            .expect("Failed to select matching release. Pre-conditions unmet, aborting test!");
-        
-        let api_result = super::create_release_attachment(
-            &client,
-            &server,
-            &repo,
-            release.id,
-            vec![String::from("Cargo.toml")],
-        )
-        .await;
+            return Self {
+                server,
+                repo,
+                release_tag,
+                client
+            };
         }
-
-    #[test]
-    fn attach_file_missing() {
-        todo!();
     }
 
     // Testing utils