selftests: net: Skip test if IPv6 is not configured

selftests: net: Skip test if IPv6 is not configured

Extend the `check_for_dependencies()` function in `lib_netcons.sh` to check
whether IPv6 is enabled by verifying the existence of
`/proc/net/if_inet6`. Having IPv6 is a now a dependency of netconsole
tests. If the file does not exist, the script will skip the test with an
appropriate message suggesting to verify if `CONFIG_IPV6` is enabled.

This prevents the test to misbehave if IPv6 is not configured.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250723-netcons_test_ipv6-v1-1-41c9092f93f9@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>