selftests: net: add test for dst hint mechanism with directed broadcast addresses

selftests: net: add test for dst hint mechanism with directed broadcast addresses

Add a test for ensuring that the dst hint mechanism is used for
directed broadcast addresses.

This test relies on mausezahn for sending directed broadcast packets.
Additionally, a high GRO flush timeout is set to ensure that packets
will be received as lists.

The test determines if the hint mechanism was used by checking
the in_brd statistic using lnstat.

Signed-off-by: Oscar Maes <oscmaes92@gmail.com>
Link: https://patch.msgid.link/20250819174642.5148-3-oscmaes92@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>