Separate folders for cpp and python tests

[arijit-dasgupta]

I have made 2 folders in tests for cpp files and python files. Plus I edited the CMakeLists.txt so that it automatically looks for every CPP and CUDA file inside of the testing folder without needing to manually add everytime.

# C++ CUDA test executable
file(GLOB CPP_CUDA_TEST_SOURCES tests/cpp_tests/*.cu tests/cpp_tests/*.cpp)
add_executable(test_add ${CPP_CUDA_TEST_SOURCES})