Added API tests + added vitest.config.ts + added fallback of window.fetch with xmlhttprequest

src/App.tsx

@@ -240,15 +240,28 @@ class App extends React.Component<IAppProps> {
  * Fetch the configuration from the API
  * @returns {Configation} The model of the configuration for the application
  */
-async function fetchConfiguration(): Promise<Configuration> {
+export async function fetchConfiguration(): Promise<Configuration> {
   const url = `${import.meta.env.VITE_API_URL}`;
 
-  return await fetch(url, {
-    method: 'POST'
-  })
-    .then((response) =>
-      response.json()
-    ) as Configuration;
+  if (window.fetch) {
+    return await fetch(url, {
+      method: 'POST'
+    })
+      .then((response) =>
+        response.json()
+      ) as Configuration;
+  }
+
+  return new Promise((resolve, reject) => {
+    const xhr = new XMLHttpRequest();
+    xhr.open('POST', url, true);
+    xhr.onreadystatechange = function() { // Call a function when the state changes.
+      if (xhr.readyState === XMLHttpRequest.DONE && xhr.status === 200) {
+        resolve(JSON.parse(this.responseText));
+      }
+    };
+    xhr.send();
+  });
 }
 
 export default App;