Fixed unittest directory check to look in the source directory

tests/CMakeLists.txt

@@ -26,7 +26,7 @@ else()
     endif()
 endif()
 
-if (NOT EXISTS tests/unittest-cpp)
+if (NOT EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/unittest-cpp)
   message(FATAL_ERROR "Could not find unittest-cpp enlistment. Please run 'git clone https://github.com/Microsoft/unittest-cpp.git unittest-cpp' in the tests directory")
 endif()