network/test_net: add support for -6 option (IPv6 tests)

* copy read_opts() from net_cmdlib.sh library;
* replace TST_EXEC_SUFFIX with more informative name TST_IPV6;

Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Reviewed-by: Cyril Hrubis <chrubis@suse.cz>
1 file changed