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

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

Introduce a synchronization mechanism between attach-f-p and strace.

* tests/attach-f-p-cmd.c: New file.
* tests/Makefile.am (check_PROGRAMS): Add attach-f-p-cmd.
* tests/attach-f-p.c: Include <sys/stat.h>.
(main): Write to stdout instead of descriptor 3.  Wait for the peer
writing to stdout.
* tests/attach-f-p.test: Assume that ./attach-f-p-cmd works.
Use $OUT for expected output.  Use attach-f-p-cmd for unlocking $OUT.