I suggest you follow how Pugs is developing. Essentially, autrijus has offered to write code to make any failing test pass that someone writes against Pugs. Here's the cool thing - by doing it this way, he guarantees that he never breaks anything that used to work.

Here's the other cool thing - he only ever is trying to make one thing work at a time. He picks a failing test and says "Ok, I'm going to work until this test passes" and does the absolute minimum to make that test pass. Then, he runs the rest of the test suite (to make sure nothing broke that used to work), then checks in the changes to SVN. Often, he'll check in 2-3 lines changes. There are usually between 10-100 checkins per day into the Pugs repository. A lot of those are new failing tests, but Autrijus makes a lot of those tests pass very quickly.