tests: always use node-fetch to simulate browser fetch (#437)

Since httpRequest is only used in the browser, it doesn't matter much
if we are using node-fetch, isomorphic-fetch, undici or node's native
fetch. Said that, to avoid tests to fail on Node 18 (which includes a
native fetch implementation), we can overwrite the global fetch with
node-fetch's implementation.

tests/unit/setup.js

@@ -14,10 +14,8 @@ const { fetch, Request } = abortableFetch({
     Request: nodeFetch.Request,
 });
 
-if (!global.fetch) {
-    global.AbortController = AbortController;
-    global.Headers = nodeFetch.Headers;
-    global.Request = Request;
-    global.Response = nodeFetch.Response;
-    global.fetch = fetch;
-}
+global.AbortController = AbortController;
+global.Headers = nodeFetch.Headers;
+global.Request = Request;
+global.Response = nodeFetch.Response;
+global.fetch = fetch;