tests/net/if_clone_test: Add a test for races between if_detach() and if_vmove_reclaim()

Ideally we shall have tests for all possible races. It is races between
if_detach(), if_vmove_loan(), if_vmove_reclaim() and vnet_if_return().
Well that requires too many tests and it appears to be less valuable to
have them all. So focus on potential in future regressions related to
recent fixes [1] and [2] only.

[1] ee9456ce3753 ifnet: Fix races in if_vmove_reclaim()
[2] ba7f47d47dc1 ifnet: if_detach(): Fix races with vmove operations

MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D56606

(cherry picked from commit f4be16983dea4904f85ac20e921ad2a8c18a0f79)
(cherry picked from commit 696d1590697e9d4c0f07ce0a8d76a62bc77bdfd5)