Add unit test for router_pick_directory_server_impl

src/or/networkstatus.c

src/or/networkstatus.h

src/or/routerlist.c

src/or/routerlist.h

src/test/include.am

src/test/test_dir.c

src/test/test_dir_common.c

src/test/test_dir_common.h

src/test/test_nodelist.c

src/test/test_routerlist.c