selftests: netconsole: Add tests for 'msgid' feature in sysdata

Extend the self-tests to cover the 'msgid' feature in sysdata.

Verify that msgid is appended to the message when the feature is enabled
and that it is not appended when the feature is disabled.

Signed-off-by: Gustavo Luiz Duarte <gustavold@gmail.com>
Reviewed-by: Breno Leitao <leitao@debian.org>
Signed-off-by: David S. Miller <davem@davemloft.net>