tests: Don't fail on kernel race conditions
tests: Don't fail on kernel race conditions

On some systems it seems to be possible to run into a case where the MAC
address of a device isn't actually set for a while.

This is likely a kernel issue as a veth pair device should always have a
MAC, but it's shown up a few times, especially in the Ubuntu CI lab and
is annoying, so let's ignore that case since it's clearly not a LXD bug.

Signed-off-by: Stéphane Graber <089afc6d81f66f1168a9849e15660feae286e024@ubuntu.com>

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