Hi Max,
There's no way to completely reset an account. However, you could simply create a new test account under your test MCC if you need a clean slate. I would not recommend that approach if your tests are going to run frequently, though, since there is a limit on the number of test accounts you can create under a test MCC.
Another option would be to remove all of the campaigns and feeds in the account. The dependent objects such as ad groups and keywords will still appear in responses, however, unless you explicitly filter them out with predicates.
Cheers,
Josh, AdWords API Team