[meteor dev here] We actually have a great realtime test system that we use to test the framework itself. Whenever you save a file, your browser test console automatically updates to show which tests are passing and failing. It's great for TDD. We also have a script that runs a set of tests in the cloud, simultaneously on all supported browsers. I'm hoping we'll get that integrated directly into the test console eventually, so you get feedback on IE breakage in realtime. The current system's API (tinytest) is sort of janky, and before we bless something as the officially recommended testing system for 1.0 I'd like to see more experience with the framework and broader community input, which is why we've held off.
Where we are weakest is on tests that are driven by UI scripting. I was really impressed with the system we had for that at Asana and I'd like to do something similar in Meteor. But right now our priorities are opening up the package system and doing a round of server scalability work.