printf: add IPv6 address format tests

printf: add IPv6 address format tests

printf_kunit already covers IPv4 address formatting, but the ip6()
test case is empty even though printk-formats.rst documents %pI6,
%pi6, %pI6c, and generic %pIS variants.

Add focused IPv6 checks for raw and generic formatting, compressed
output, the single-zero %pI6c corner case, and bracketed port
formatting for sockaddr_in6.

Signed-off-by: Shuvam Pandey <shuvampandey1@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Tested-by: Petr Mladek <pmladek@suse.com>
Link: https://patch.msgid.link/20260317114548.98919-1-shuvampandey1@gmail.com
[pmladek@suse.com: Removed non-necessary details from the commit message.]
Signed-off-by: Petr Mladek <pmladek@suse.com>