refine math_function_gpu_test and selected_rows_functor_gpu_test
Created by: luotao1
Inspired by https://github.com/PaddlePaddle/Paddle/pull/8682#discussion_r171779349, we should refine math_function_test.cc and selected_rows_functor_test.cc: https://github.com/PaddlePaddle/Paddle/blob/60e7ee0611745f07f1a90d32e6253f9c4161eaee/paddle/fluid/operators/math/CMakeLists.txt#L5-L7
A better test example is im2col_test.cc and sequence_padding_test.cc.