Test cases using iframes for issue #12752. Currently don't work.

I tried to write test cases for pull request #12759 using `<iframe>`, as suggested there by @JamesMGreene. They don't work, but I can't figure out why.  The `--verbose` output for both of the new tests is

    got here 1
    got here 2
    Process is running more than 10 seconds. Terminating...

I suspect the script in `iframe.html` is not doing what it's supposed to, but I haven't been able to get any debugging output from there, and I need to be doing other things.  Perhaps someone else can take it from here.