tests: enhance nlattr_inet_diag_msg test

tests: enhance nlattr_inet_diag_msg test

* tests/test_nlattr.h: New file.
* tests/Makefile.am (libtests_a_SOURCES): Add it.
* tests/nlattr_inet_diag_msg.c: Include "test_nlattr.h".
(test_inet_diag_meminfo, test_inet_diag_vegasinfo,
test_inet_diag_dctcpinfo, test_inet_diag_bbrinfo): Remove.
(address): New variable.
(init_inet_diag_msg): Remove "address" argument, add const qualifier
to all remaining arguments.
(print_inet_diag_msg, print_uint): New functions.
(main): Use macros from test_nlattr.h file.