Add initial set of Playwright integration tests, for testing embeds (#254)

* add first pass on playwright integration tests!
- test embed loading via localhost:9990/embed.html as well as cross-domain with embed on localhost:8020/
while RWP + data on localhost:9990/
- add --ignore-optional to most ci runs
- add playwright ci test
- add playwright and playwright-test to dev dependencies
- add 'test-start-embed' to start test harness for localhost:8020/ embed
- add 'test-start-sandbox' to start test harndess for localhost:8030/ sandbox
- add additional tests for loading with sandbox + requiresubdomainiframe, ensure 'requiresubdomainiframe' works, doesn't allow loading directly

tests/embed/index-sandbox.html

@@ -0,0 +1,19 @@
+<!doctype html>
+<html>
+<head>
+<script src="http://localhost:9990/ui.js"></script>
+<style>
+html, body {
+  background-color: lightgrey;
+  height: 100%;
+  width: 100%;
+  overflow: hidden;
+}
+</style>
+</head>
+<body>
+
+<replay-web-page embed="replay" src="http://localhost:9990/docs/assets/tweet-example.wacz" url="page:0" sandbox requiresubdomainiframe></replay-web-page>
+
+</body>
+</html>