[CI]Added initial framework of CXX unittest (#98)

Based on the discussion in #53 , I just added the initial flow of CXX unittests for this repo, with providing two dummy UTs as placeholder to show the usage, feel free to add your own CXX unittests. 
@Superjomn  @ptillet 

@ptillet , in this PR, I also configure the integration-tests.yml to add the unittest into github CI check. 

Thanks

CMakeLists.txt

@@ -220,3 +220,5 @@ if(BUILD_PYTHON_MODULE AND NOT WIN32)
 endif()
 
 add_subdirectory(test)
+
+add_subdirectory(unittest)