[web snapshot] Refactor mjsunit tests

This CL refactors mjsunit tests, so that the common core of all tests is
abstracted away.

Bug: v8:11525, v8:11706
Change-Id: I24a1af4298380e21a64e4d17149422c32fbf8a4d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2914882Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Vicky Kontoura <vkont@google.com>
Cr-Commit-Position: refs/heads/master@{#74853}

test/mjsunit/web-snapshot/web-snapshot.js

@@ -5,28 +5,37 @@
 // Flags: --experimental-d8-web-snapshot-api
 
 function callString(f) {
-    return '(' + f.toString() + ')()';
+  return '(' + f.toString() + ')()';
 }
 
-(function TestMinimal() {
-    const r1 = Realm.create();
-    function initialize() {
-        globalThis.foo = {
-            'str': 'hello',
-            'n': 42,
-        };
-    }
-    Realm.eval(r1, callString(initialize));
-    const snapshot = Realm.takeWebSnapshot(r1, ['foo']);
+function use() {
+  const result = Object.create(null);
+  Realm.shared.exports.forEach(x => result[x] = globalThis[x]);
+  return result;
+}
 
-    const r2 = Realm.create();
-    function use() {
-        return globalThis.foo;
-    }
-    const success = Realm.useWebSnapshot(r2, snapshot);
-    assertTrue(success);
+function takeAndUseWebSnapshot(createObjects, exports) {
+  // Make the exports list available across Realms.
+  Realm.shared = { exports };
+  // Take a snapshot in Realm r1.
+  const r1 = Realm.create();
+  Realm.eval(r1, callString(createObjects));
+  const snapshot = Realm.takeWebSnapshot(r1, exports);
+  // Use the snapshot in Realm r2.
+  const r2 = Realm.create();
+  const success = Realm.useWebSnapshot(r2, snapshot);
+  assertTrue(success);
+  return Realm.eval(r2, callString(use));
+}
 
-    const foo = Realm.eval(r2, callString(use));
-    assertEquals(foo.str, 'hello');
-    assertEquals(foo.n, 42);
+(function TestMinimal() {
+  function createObjects() {
+    globalThis.foo = {
+      str: 'hello',
+      n: 42,
+    };
+  }
+  const exports = takeAndUseWebSnapshot(createObjects, ['foo']);
+  assertEquals(exports.foo.str, 'hello');
+  assertEquals(exports.foo.n, 42);
 })();