Add multi browser tests + wabac.js update (#420)

Fidelity: Fix CSP issue with iframes in Firefox (via wabac.js 2.23.0)
Loading: Avoid unnecessary reloads of multiWACZ JSON, prune expired
multiWACZ collections

Testing improvements:
- add playwright multi browser tests with firefox and webkit
- add test of iframe replay with iframetest.wacz

deps: update to wabac.js 2.23.0
bump to 2.3.9

tests/embed/iframe-test-cross.html

@@ -0,0 +1,19 @@
+<!doctype html>
+<html>
+  <head>
+    <script src="http://localhost:9990/ui.js"></script>
+    <style>
+      body {
+        background-color: lightgrey;
+        height: 600px;
+      }
+    </style>
+  </head>
+  <body>
+    <replay-web-page
+      noWebWorker
+      src="http://localhost:9990/docs/examples/iframetest.wacz"
+      url="page:0"
+    ></replay-web-page>
+  </body>
+</html>