Enable -Wextra in tests.

tests/Makefile

tests/ctype.c

tests/errno.c

tests/futimens.c

tests/grp/gr_iter.c

tests/libgen.c

tests/pthread/cleanup.c

tests/pthread/extjoin.c

tests/pthread/main.c

tests/ptrace.c

tests/signal.c

tests/stdio/fread.c

tests/stdio/freopen.c

tests/stdlib/alloc.c

tests/stdlib/qsort.c

tests/stdlib/strtod.c

tests/sys_epoll/epoll.c

tests/time/localtime.c

tests/tls.c

tests/unistd/getopt.c