test: fix cookie tests on WebKit (#2359)

WebKit does not allow third party cookies.

Author: dgozman

test/defaultbrowsercontext.spec.js

@@ -111,9 +111,9 @@ describe('launchPersistentContext()', function() {
       document.cookie = 'username=John Doe';
       return document.cookie;
     });
-    expect(documentCookie).toBe('username=John Doe');
     await page.waitForTimeout(2000);
     const allowsThirdParty = CHROMIUM || FFOX;
+    expect(documentCookie).toBe(allowsThirdParty ? 'username=John Doe' : '');
     const cookies = await context.cookies(server.CROSS_PROCESS_PREFIX + '/grid.html');
     if (allowsThirdParty) {
       expect(cookies).toEqual([

test/headful.spec.js

@@ -96,9 +96,9 @@ describe('Headful', function() {
       document.cookie = 'username=John Doe';
       return document.cookie;
     });
-    expect(documentCookie).toBe('username=John Doe');
     await page.waitForTimeout(2000);
     const allowsThirdParty = CHROMIUM || FFOX;
+    expect(documentCookie).toBe(allowsThirdParty ? 'username=John Doe' : '');
     const cookies = await page.context().cookies(server.CROSS_PROCESS_PREFIX + '/grid.html');
     if (allowsThirdParty) {
       expect(cookies).toEqual([