Fix: tests: missing _GNU_SOURCE for F_GETPIPE_SZ

Fix: tests: missing _GNU_SOURCE for F_GETPIPE_SZ

Per man 2 fcntl:

  F_GETOWN_EX,  F_SETOWN_EX,  F_SETPIPE_SZ,  F_GETPIPE_SZ,  F_GETSIG,  F_SETSIG,
  F_NOTIFY,   F_GETLEASE,   and  F_SETLEASE  are  Linux-specific.   (Define  the
  _GNU_SOURCE macro to obtain these definitions.)

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I2b61dfb79ffa384dc2bab56cd3510ddc6ae21e85