tests: move common ifinfomsg boilerplate into a separate header

tests: move common ifinfomsg boilerplate into a separate header

* configure.ac (AC_CHECK_DECLS): Check for IFLA_PORT_SELF, IFLA_PROTINFO, and
IFLA_XDP in <linux/rtnetlink.h>.
* tests/nlattr_ifla.h: New file.
* tests/Makefile.am (EXTRA_DIST): Add it.
* tests/nlattr_ifla_brport.c [!HAVE_DECL_IFLA_PROTINFO] (IFLA_PROTINFO):
New declaration.
(hdrlen, init_ifinfomsg, print_ifinfomsg): Remove.
(IFLA_ATTR): Define to IFLA_PROTINFO.
Include nlattr_ifla.h.
* tests/nlattr_ifla_port.c [!HAVE_DECL_IFLA_PORT_SELF] (IFLA_PORT_SELF):
New declaration.
(hdrlen, init_ifinfomsg, print_ifinfomsg): Remove.
(IFLA_ATTR): Define to IFLA_PORT_SELF.
Include nlattr_ifla.h.
* tests/nlattr_ifla_xdp.c
* tests/nlattr_ifla_xdp.c [!HAVE_DECL_IFLA_XDP] (IFLA_XDP): New
declaration.
(hdrlen, init_ifinfomsg, print_ifinfomsg): Remove.
(IFLA_ATTR): Define to IFLA_XDP.
Include nlattr_ifla.h.