modified test_concat_layer.cpp

The way of using the `ConstantFiller` class in `src/caffe/test/test_concat_layer.cpp` seems to be not valid. Detail information is referred to https://github.com/zszhong/caffe/tree/test/test/test_filler.
Thus I modified the `src/caffe/test/test_concat_layer.cpp` as that in the `src/caffe/test/test_convolution_layer.cpp`.