selftests: net: add netconsole test for cmdline configuration

Add a new selftest to verify netconsole module loading with command
line arguments. This test exercises the init_netconsole() path and
validates proper parsing of the netconsole= parameter format.

The test:
- Loads netconsole module with cmdline configuration instead of
  dynamic reconfiguration
- Validates message transmission through the configured target
- Adds helper functions for cmdline string generation and module
  validation

This complements existing netconsole selftests by covering the
module initialization code path that processes boot-time parameters.
This test is useful to test issues like the one described in [1].

Link: https://lore.kernel.org/netdev/Z36TlACdNMwFD7wv@dev-ushankar.dev.purestorage.com/ [1]
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20250613-rework-v3-8-0752bf2e6912@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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