tests: check path tracing of ppoll syscall

tests: check path tracing of ppoll syscall

* tests/ppoll.c (main) [PATH_TRACING_FD]: Skip if /proc/self/fd/
is not available.  Add a test call that use PATH_TRACING_FD.
(main): Conditionalize expected output for those calls
that do not use PATH_TRACING_FD with [!PATH_TRACING_FD].
* tests/ppoll-P.c: New file.
* tests/pure_executables.list: Add ppoll-P.
* tests/.gitignore: Likewise.
* tests/gen_tests.in (ppoll-P): New entry.