We should add (will need some adapting) Microsofts donated ES5 test suite to LayoutTests. It can be found at http://wiki.ecmascript.org/doku.php?id=es3.1:es3.1_proposal_working_draft. Obviously not all will pass yet (as we have not completed ES5) but it will still be good to have the tests there.
I think the test suite is also rather incomplete at this point. It'd definitely be good to get it in, but we should keep an eye on the upstream version after doing so in order to keep our versions up to date.
I have a python version of their harness generating script that makes the harness usable by us. I'd like to do some more work to make it JS only however rather than requiring us run it in the browser.