Boost unit test execution speed.

Per-test forking is unnecessary and quite expensive
given that we do so little in each test.
Exposing JsonConfig used for property setting.

JsonConfig change the way the value is mangled, so this allows the
caller of the builder to tweak the behaviour.

(The reason I needed this change was because I use different default
values for some of the JsonConfig options, and this allows me to keep
tests happy with minimal modification. That said, I think this feature
makes sense on its own.)
Merge pull request #12 from kohsuke/easy-changes

Two low-hanging fruits from the Jenkins patch set