Single file tests

So one request I've had from people at Mozilla used to writing 
Mochitests is an easier way to write testharness.js tests, particularly 
for the special case where there is exactly one test per file. This is a 
interesting case because the file itself provides the isolation that we 
usually try to obtain by wrapping each test step in a function() {}, so 
it is possible to cut out some of the verbosity. An example of the kind 
of result we might get is

<doctype html>
<title>Example test</title>
<script src=/resources/testharness.js></script>
<script src=/resources/testharnessreport.js></script>
<script>
setup({file_is_test:true})

onload = function() {assert_true(true); done()}

I think there are some possible disadvantages to this; in particular it 
might encourage people to put a whole load of things that should be 
separate tests as multiple asserts in one file in a way that will break 
if one assert doesn't work in a particular browser. However it does seem 
like if used wisely it could be a win, and might help sell the idea of 
writing testharness.js tests going forward at Mozilla. What do people 
think?