selftests: netconsole: Add support for basic netconsole target format

Extend the netconsole selftest to validate both basic and extended
target formats. The basic format is a simpler variant that doesn't
support userdata or release functionality.

The test now validates that netconsole works correctly in both
configurations, improving test coverage for different netconsole
deployment scenarios.

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20250609-netcons_ext-v3-4-5336fa670326@debian.org


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