Congratulations. The assessment test is probably no big deal, especially if they provide 2 weeks of training for the job that you will actually be expected to perform. Seriously, it's not like hanging wire bundles in planes is rocket science or anything. They likely use the test to gauge your level of overall mechanical inclination, aptitude, ability to read blueprints, etc.
Despite the moaners and bitchers above, Boeing is still a decent place to work. The money doesn't go quite as far as it used to in this area, but it's still a comfortable living. The benefits are pretty good as well. If you aren't aware, the commercial aircraft biz is cyclical, so Boeing does lay off periodically, like every 8-10 years or so. When I worked there, all the older guys said to plan on being laid off probably once. But then my wife has worked there for 20 years now and has never even come close to being laid off. And Boeing is facing the same situation as many other companies: an aging workforce with a considerable number of folks looking at retirement in the not too distant future. They haven't hired off the street much recently, so getting your foot in the door now might mean being able to avoid the next round of layoffs in another 6-8 years.