tests: check path tracing of poll syscall

tests: check path tracing of poll syscall

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