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

[zlei]

Summary

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

[1] ee9456ce3753 ifnet: Fix races in if_vmove_reclaim()
[2] D56374 ifnet: if_detach(): Fix races with if_vmove()

MFC after: 5 days

Test Plan

Current requires extra effort du to virtualization of bpf. Run against stable/15 or stable/14 instead.

[zlei]

tests/sys/net/if_clone_test.sh

125

This test will surely cleanup the clone created epair interfaces by it self. Seems no need for cleanup_ifaces. Please correct me if wrong.