use method overrides for CuDNNPoolingLayer top blob checking
cuDNN pooling layer tests know that nonzero padding is not supported

After #1172, some cuDNN pooling layer tests were failing due to use of
padding. (Before #1172, these tests were actually testing PoolingLayer
instead of CuDNNPoolingLayer via the fallback.) This commit disables
many tests via commenting, so that they can be easily readded when cuDNN
gains pooling padding support.
Merge pull request #1187 from longjon/fix-cudnn-pooling-tests

Fix cuDNN pooling tests