kselftests/net: list all available tests in usage()

So that users can run/query them easily.

$ ./fcnal-test.sh -h
usage: fcnal-test.sh OPTS

	-4          IPv4 tests only
	-6          IPv6 tests only
	-t <test>   Test name/set to run
	-p          Pause on fail
	-P          Pause after each test
	-v          Be verbose

Tests:
	ipv4_ping ipv4_tcp ipv4_udp ipv4_bind ipv4_runtime ipv4_netfilter ipv6_ping ipv6_tcp ipv6_udp ipv6_bind ipv6_runtime ipv6_netfilter use_cases

Suggested-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>