register_tests now takes working directory.

usage:
register_tests(SOURCES
                 ${list_of_source_files}
                 "test_src.cc"
                 ${concatenated_string_of_source_files}
               NAMESPACE
                 "test_namespace"
               LINK_LIBRARIES
                 ${list_of_targets}
               INCLUDE_DIRECTORIES
                 ${list_of_include_directories}
               COMPILE_OPTIONS
                 ${list_of_compile_options}
               WORKING_DIRECTORY
                 ${ctest_working_directory})