xxx.h, xxx.{cc,cu}, xxx_test.{cc,cu} in the same directory

[wangkuiyi]

As we are adopting the new building system, whose design doc is at here, we should build each unit test into a standalone executable binary file. So we don't have to put all unit tests of a C++ package into a test directory. Instead; we can put all three files of each module together in the same directory.