tests: robustify attach-p-cmd.test against race conditions

tests: robustify attach-p-cmd.test against race conditions

Introduce a synchronization mechanism between attach-p-cmd-p
and attach-p-cmd-cmd processes.

* tests/attach-p-cmd-cmd.c: Include <errno.h> and <sys/stat.h>.
(main): Wait for the lock directory creation by attach-p-cmd-p.
* tests/attach-p-cmd-p.c: Include <errno.h> and <sys/stat.h>.
(main): Create a lock directory and wait for its removal
by attach-p-cmd-cmd.
* tests/attach-p-cmd.test: Assume that test programs work.