refactor some re-usable methods into LiteTestCase

https://github.com/couchbase/couchbase-lite-android/issues/398
trying to get mockSinglePull test working

https://github.com/couchbase/couchbase-lite-android/issues/398
make Puller a subclass of ReplicationInternal

https://github.com/couchbase/couchbase-lite-android/issues/398
add assertion

https://github.com/couchbase/couchbase-lite-android/issues/398