common: Add test case for IPC with spawned process.
* common/Makefile.am (module_tests): Add t-exechelp. * common/t-exechelp.c [HAVE_W32_SYSTEM] (print_open_fds) (test_close_all_fds, main): Exclude the test_close_all_fds test. (run_server, test_pipe_stream): New.
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>