tests: regenerate eth2 veth after HSR tests

nmstate copy_hsr_mac() sets cloned-mac-address on HSR PRP port
NM profiles but does not clean it up on removal, leaving eth1
and eth2 with identical MACs for the rest of the test session.
This causes ~50% failure rate in test_description_on_down_interface_ref_by_mac.

Add a module-scoped autouse fixture to hsr_test.py that regenerates
the eth2 veth pair after HSR tests, restoring a unique MAC.

Signed-off-by: Vladimír Beneš <vbenes@redhat.com>