netconsole: selftest: Split the helpers from the selftest

netconsole: selftest: Split the helpers from the selftest

Split helper functions from the netconsole basic test into a separate
library file to enable reuse across different netconsole tests. This
change only moves the existing helper functions to lib/sh/lib_netcons.sh
while preserving the same test functionality.

The helpers provide common functions for:
- Setting up network namespaces and interfaces
- Managing netconsole dynamic targets
- Setting user data
- Handling test dependencies
- Cleanup operations

Do not make any change in the code, other than the mechanical
separation.

Signed-off-by: Breno Leitao <leitao@debian.org>
Tested-by: Simon Horman <horms@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250108-netcons_overflow_test-v3-2-3d85eb091bec@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>