Fix sketchy label initialization in NetTest

`NetTest`'s `TinyNet` uses a softmax loss layer, but initializes its labels with a zero-mean Gaussian(!). This means that labels are effectively (usually) either 0 or (invalidly) -1.

This PR add a `data_filler` to make the labels always zero. Note that `TrickyNet` also uses this setup, but never calls forward or backward, so this is not an issue.