tests can fail in ways that leave lingering simnet interfaces

[rcgoodfellow]

running 1 test
test test_v6_local_lifecycle ... FAILED

failures:

---- test_v6_local_lifecycle stdout ----
thread 'test_v6_local_lifecycle' panicked at 'get info: Ioctl("socaddr to ipaddr conversion")', libnet/tests/ip.rs:107:10
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
deleting lnt_v6ls_sim3 failed: ioctl: ioctl @fd.as_raw_fd()=3 sys::SIMNET_IOC_DELETE: Device busy (os error 16)

We can bring in the falcon ztest machinery in to make this better.