Add unit tests for select operations
Add unit tests for select operations


Former-commit-id: fcc5d115b320e37f10ba8b64a74d000e3ce4c7da
chunyang-wen/dynet,danielhers/dynet,clab/dynet,clab/cnn,xunzhang/dynet,shuheik/dynet,shuheik/dynet,clab/dynet,xunzhang/dynet,clab/cnn,danielhers/dynet,clab/dynet,shuheik/dynet,chunyang-wen/dynet,danielhers/dynet,clab/cnn,shuheik/dynet,clab/dynet,shuheik/dynet,clab/dynet,danielhers/dynet,chunyang-wen/dynet,danielhers/dynet,danielhers/dynet,shuheik/dynet,xunzhang/dynet,chunyang-wen/dynet,clab/dynet,clab/dynet,shuheik/dynet,danielhers/dynet,xunzhang/dynet