add mockHeaders test util

frontend/app/common/fetcher.test.ts

@@ -1,23 +1,13 @@
 import fetcher from './fetcher';
+import { mockHeaders } from '@app/testUtils/mockHeaders';
 
 describe('fetcher', () => {
-  let originalHeaders = (window as any).Headers;
-
   beforeAll(() => {
-    originalHeaders = (window as any).Headers;
-    (window as any).Headers = class {
-      append() {}
-      has() {
-        return false;
-      }
-      get() {
-        return null;
-      }
-    };
+    mockHeaders.mock();
   });
 
   afterAll(() => {
-    (window as any).Headers = originalHeaders;
+    mockHeaders.restore();
   });
 
   afterEach(() => {

frontend/app/testUtils/mockHeaders.ts

@@ -0,0 +1,21 @@
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+const originalHeaders = (window as any).Headers;
+
+export const mockHeaders = {
+  mock: () => {
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    (window as any).Headers = class {
+      append() {}
+      has() {
+        return false;
+      }
+      get() {
+        return null;
+      }
+    };
+  },
+  restore: () => {
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    (window as any).Headers = originalHeaders;
+  },
+};