Base: Add a test for the early return steps of "prepare a script"

This tests the early return steps of "prepare a script" that come
_before_ step 10 "Set the element's "already started" flag". The
relevant steps are steps 6, 7 and 8. If this algorithm returns on any
of these steps, the script can be reinserted matching the requirements
and will run.

https://html.spec.whatwg.org/multipage/scripting.html#prepare-a-script

I wrote this test page up while testing something else, but found a bug
in Firefox where it doesn't allow re-preparing the script if step 8
fails: https://bugzilla.mozilla.org/show_bug.cgi?id=1735590

Base/res/html/misc/welcome.html

@@ -143,6 +143,7 @@
             <li><a href="demo.html">fun demo</a></li>
             <li><a href="set-interval.html">setInterval()</a></li>
             <li><a href="location.html">window.location property</a></li>
+            <li><a href="script-preparation-test.html">Test for the early return steps 6-8 of the "prepare a script" algorithm</a></li>
             <li><h3>Canvas</h3></li>
             <li><a href="canvas.html">canvas 2D test</a></li>
             <li><a href="canvas-rotate.html">canvas rotate()</a></li>