selftests: net: add test for destination in broadcast packets

selftests: net: add test for destination in broadcast packets

Add test to check the broadcast ethernet destination field is set
correctly.

This test sends a broadcast ping, captures it using tcpdump and
ensures that all bits of the 6 octet ethernet destination address
are correctly set by examining the output capture file.

Co-developed-by: Brett A C Sheffield <bacs@librecast.net>
Signed-off-by: Brett A C Sheffield <bacs@librecast.net>
Signed-off-by: Oscar Maes <oscmaes92@gmail.com>
Link: https://patch.msgid.link/20250902150240.4272-1-oscmaes92@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>