Fix WORKING_DIRECTORY for unit tests

When jsoncons was fetched from another project, ctest failed because the path wasn't found

test/CMakeLists.txt

@@ -215,7 +215,7 @@ target_compile_options(unit_tests PRIVATE
 )
 endif()
 
-add_test(NAME unit_tests COMMAND unit_tests WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}/test)
+add_test(NAME unit_tests COMMAND unit_tests WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})
 
 target_include_directories (unit_tests 
                             PUBLIC ${JSONCONS_INCLUDE_DIR}