selftests: Add a test of large binary to devlink health test

Add a test of 2 PAGEs size (exceeds devlink previous length limitation)
of binary data on a 'devlink health dump show' command. Set binary length
to 8192, issue a dump show command and clear it.

Signed-off-by: Aya Levin <ayal@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>