[PATCH] selftests: rtnetlink: add checks for ifconfig and iproute2

Hi,

This patch improves portability of the rtnetlink selftests in two ways:

1. It wraps a call to ifconfig in a presence check to avoid test failures
   on systems where ifconfig is not installed — such as default Debian Bookworm
   and newer distributions where iproute2 is the norm.

2. It skips the do_test_address_proto test if the installed version of iproute2
   does not support the proto in ip address commands. Without this check,
   the test fails unconditionally on older iproute2 versions, even though the kernel
   functionality under test is not the culprit.

Both changes ensure that the test suite degrades gracefully by reporting SKIP
instead of FAIL on incompatible systems.

Tested on Debian Bookworm with iproute2 6.1.0 and without ifconfig.

Thanks for your time and consideration.

Best regards,
Alessandro Ratti