CMakeLists.txt:
- Added the find_package command and added the GTEST_INCLUDE_DIRS to the include directories.

UnitTests/CMakeLists.txt:
- Removed the old add_library command, because it doesn’t transport the compiler flags that must be used when building your own tests and it doesn’t bring in dependent libraries with the target (like pthreads in this case).
- Using variables and targets brought in by the find_package command from the parent CMakeLists.txt file.