Added some more tests (XAMPPRocky#578)

- `repos().issues().check_assignee()`
  - Covers 204 case
  - Covers 404 case
  - Covers 500 case

tests/issues_check_assignee_test.rs

@@ -0,0 +1,95 @@
+mod mock_error;
+
+use mock_error::setup_error_handler;
+use octocrab::Octocrab;
+use wiremock::{
+    matchers::{method, path},
+    Mock, MockServer, ResponseTemplate,
+};
+
+async fn setup_issue_check_assignee_api(template: ResponseTemplate) -> MockServer {
+    let owner: &str = "org";
+    let repo: &str = "some-repo";
+    let assignee: &str = "some-user";
+
+    let mock_server = MockServer::start().await;
+
+    Mock::given(method("GET"))
+        .and(path(format!("/repos/{owner}/{repo}/assignees/{assignee}")))
+        .respond_with(template.clone())
+        .mount(&mock_server)
+        .await;
+
+    setup_error_handler(
+        &mock_server,
+        &format!("GET on /repos/{owner}/{repo}/assignees/{assignee} was not received"),
+    )
+    .await;
+    mock_server
+}
+
+fn setup_octocrab(uri: &str) -> Octocrab {
+    Octocrab::builder().base_uri(uri).unwrap().build().unwrap()
+}
+
+const OWNER: &str = "org";
+const REPO: &str = "some-repo";
+const ASSIGNEE: &str = "some-user";
+
+#[tokio::test]
+async fn check_assignee_204() {
+    let template = ResponseTemplate::new(204);
+    let mock_server = setup_issue_check_assignee_api(template).await;
+    let client = setup_octocrab(&mock_server.uri());
+
+    let result = client
+        .issues(OWNER.to_owned(), REPO.to_owned())
+        .check_assignee(ASSIGNEE)
+        .await;
+
+    assert!(
+        result.is_ok(),
+        "expected successful result, got error: {:#?}",
+        result
+    );
+    let result = result.unwrap();
+    assert!(result, "expected the result to be true: {}", result);
+}
+
+#[tokio::test]
+async fn check_assignee_404() {
+    let template = ResponseTemplate::new(404);
+    let mock_server = setup_issue_check_assignee_api(template).await;
+    let client = setup_octocrab(&mock_server.uri());
+
+    let result = client
+        .issues(OWNER.to_owned(), REPO.to_owned())
+        .check_assignee(ASSIGNEE)
+        .await;
+
+    assert!(
+        result.is_ok(),
+        "expected successful result, got error: {:#?}",
+        result
+    );
+    let result = result.unwrap();
+    assert!(!result, "expected the result to be false: {}", result);
+}
+
+#[tokio::test]
+async fn check_assignee_500() {
+    let template = ResponseTemplate::new(500);
+    let mock_server = setup_issue_check_assignee_api(template).await;
+    let client = setup_octocrab(&mock_server.uri());
+
+    let result = client
+        .issues(OWNER.to_owned(), REPO.to_owned())
+        .check_assignee(ASSIGNEE)
+        .await;
+
+    assert!(
+        result.is_err(),
+        "expected error result, got success: {:#?}",
+        result
+    );
+}