selftests/net/openvswitch: add SET action test

selftests/net/openvswitch: add SET action test

Add test_action_set exercising OVS_ACTION_ATTR_SET with an ipv4 dst
rewrite. The test verifies the SET action in three steps: first
confirm normal forwarding, then apply set(ipv4(dst=10.0.0.99)) to
rewrite the destination to an address nobody owns and verify ping
fails, then restore normal forwarding and verify connectivity
recovers.

Signed-off-by: Minxi Hou <houminxi@gmail.com>
Reviewed-by: Aaron Conole <aconole@redhat.com>
Link: https://patch.msgid.link/20260612130503.311240-1-houminxi@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>