net: netconsole: selftests: Add userdata validation

Extend netcons_basic selftest to verify the userdata functionality by:
 1. Creating a test key in the userdata configfs directory
 2. Writing a known value to the key
 3. Validating the key-value pair appears in the captured network output

This ensures the userdata feature is properly tested during selftests.

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20241029090030.1793551-3-leitao@debian.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>