Add a reduced test-case for bug 1027533

When fixing [bug 1027533](https://bugzilla.mozilla.org/show_bug.cgi?id=1027533) in PR #4980, I didn't bother to include a test-case. This meant that while doing the re-factoring for PR&nbsp;#5095, I had to manually check that I didn't regress anything.
So this patch does what I *should* have done in the first place; by adding a reduced test-case for [bug 1027533](https://bugzilla.mozilla.org/show_bug.cgi?id=1027533).