Fix cuDNN pooling tests

#1172 broke cuDNN pooling tests in dev. Actually, cuDNN pooling tests were largely testing Caffe's `PoolingLayer` due to the padding fallback before #1172. These commits comment the cuDNN pooling tests that rely on padding, and modify the ones that used it but didn't need it.

I've checked that this passes build, lint, warn, and runtest with `USE_CUDNN`. For now, let's remember to `make everything` or equivalent when merging changes to cuDNN.