[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

unittest/googletest.cmake

@@ -0,0 +1,23 @@
+include(FetchContent)
+
+set(GOOGLETEST_DIR "" CACHE STRING "Location of local GoogleTest repo to build against")
+
+if(GOOGLETEST_DIR)
+  set(FETCHCONTENT_SOURCE_DIR_GOOGLETEST ${GOOGLETEST_DIR} CACHE STRING "GoogleTest source directory override")
+endif()
+
+FetchContent_Declare(
+  googletest
+  GIT_REPOSITORY https://github.com/google/googletest.git
+  GIT_TAG release-1.12.1
+  )
+
+FetchContent_GetProperties(googletest)
+
+if(NOT googletest_POPULATED)
+  FetchContent_Populate(googletest)
+  if (MSVC)
+    set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
+  endif()
+  add_subdirectory(${googletest_SOURCE_DIR} ${googletest_BINARY_DIR} EXCLUDE_FROM_ALL)
+endif()