feat(Response): add Response.fromCache / Response.fromServiceWorker (#1971)

This patch:
- introduces `test/assets/cached` folder and teaches server to cache
  all the assets from the folder
- introduces `test/assets/serviceworkers` folder that stores all the
  service workers and makes them register with unique URL prefix
- introduces `Response.fromCache()` and `Response.fromServiceWorker()`
  methods

Fixes #1551.

test/assets/cached/one-style.css

@@ -0,0 +1,3 @@
+body {
+  background-color: pink;
+}

test/assets/cached/one-style.html

@@ -0,0 +1,2 @@
+<link rel='stylesheet' href='./one-style.css'>
+<div>hello, world!</div>

test/assets/serviceworkers/empty/sw.html

@@ -0,0 +1,3 @@
+<script>
+  window.registrationPromise = navigator.serviceWorker.register('sw.js');
+</script>

test/assets/serviceworkers/fetch/style.css

@@ -0,0 +1,3 @@
+body {
+    background-color: pink;
+}

test/assets/serviceworkers/fetch/sw.html

@@ -0,0 +1,4 @@
+<link rel="stylesheet" href="./style.css">
+<script>
+  navigator.serviceWorker.register('sw.js');
+</script>

test/assets/serviceworkers/fetch/sw.js

@@ -0,0 +1,3 @@
+self.addEventListener('fetch', event => {
+  event.respondWith(fetch(event.request));
+});