net: netconsole: selftests: Check if netdevsim is available

The netconsole selftest relies on the availability of the netdevsim module.
To ensure the test can run correctly, we need to check if the netdevsim
module is either loaded or built-in before proceeding.

Update the netconsole selftest to check for the existence of
the /sys/bus/netdevsim/new_device file before running the test. If the
file is not found, the test is skipped with an explanation that the
CONFIG_NETDEVSIM kernel config option may not be enabled.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20241108-netcon_selftest_deps-v1-1-1789cbf3adcd@debian.org


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