Some testharness.js comments

Hi James,

a few comments for your consideration.

This one will fail as expected but no relevant message is generated in
the log:
[[

function throwException() { throw "Exception"; }

test(function() { assert_true(throwException()); }, "throw an exception
within a test");

]]


This one seems to fail on Firefox 4:
[[

test(function() { assert_exists(document, "documentElement"); },
"document.documentElement exists");

]]

It would also be nice to be able to write:
[[

test(function() { assert_exists(document, "createElement"); },
"document.createElement exists");

]]

Thank you,

Philippe