selftests/coredump: add tests for AF_UNIX coredumps

Add a simple test for generating coredumps via AF_UNIX sockets.

Link: https://lore.kernel.org/[email protected]
Acked-by: Luca Boccassi <[email protected]>
Reviewed-by: Alexander Mikhalitsyn <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

Author: brauner