I wouldn't worry about it being sweet at all. If you inadvertently boiled your flavor hops for a longer time, if anything you'll get more bitterness.
You probably did lose flavor, though, since the compounds that provide flavor are quite volatile and boil off quickly.
Dry-hopping provides lots of hops flavor and aroma, so you might wish to try it. If your recipe called for such a small amount of Tettnanger as flavor/aromatic hops in the boil, however, it will be really hard to avoid overdoing it. Perhaps you can add 1/8 oz to the secondary/lagerkeller, give it a week, taste it, and if you think it needs more add the other 1/8 oz. Better to sneak up on the flavor and aroma you're after than go too far all at once, 'cos once it's in there you can't get it out.