Make test_net use DUMMY_DATA instead of DATA (leveldb)

This PR changes test_net.cpp to use the new `DUMMY_DATA` rather than the `DATA` layer.  This is fine since test_net.cpp tests nothing pertinent to the data layer or leveldb itself (unlike, say, the data layer tests), so it's a waste of time to create one.  The net tests now take roughly 1ms each, instead of the ~250ms each they took before.  (obviously not a huge deal to begin with, but still an improvement, and serves as an example of using DummyDataLayer)