selftests: net: validate team flags propagation

selftests: net: validate team flags propagation

Cover three recent cases:
1. missing ops locking for the lowers during netdev_sync_lower_features
2. missing locking for dev_set_promiscuity (plus netdev_ops_assert_locked
   with a comment on why/when it's needed)
3. rcu lock during team_change_rx_flags

Verified that each one triggers when the respective fix is reverted.
Not sure about the placement, but since it all relies on teaming,
added to the teaming directory.

One ugly bit is that I add NETIF_F_LRO to netdevsim; there is no way
to trigger netdev_sync_lower_features without it.

Signed-off-by: Stanislav Fomichev <stfomichev@gmail.com>
Link: https://patch.msgid.link/20250516232205.539266-1-stfomichev@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>