Make DummyObservable visible to clojureTest classpath.

RxJava issue #327. Moved DummyObservable into main so that it's
compiled and available when the clojureTest task executes. Perhaps
someone with stronger Gradle skills can figure out how to put this Java
class in src/test and get things to work out. Correctly. For the time
being, several clean/build cycles worked fine for me.