Retire the three ignored Ethernet tests, which covered nothing unique

All three required root or CAP_NET_RAW, neither runner passes --ignored,
and so none had ever executed in CI. Before deleting them I checked what
each covered and whether anything else covered it:

- The two-node handshake is covered by the ethernet-only chaos scenario,
  whose baseline assertion cannot pass unless handshakes complete over
  AF_PACKET.
- Tree convergence and root election by smallest NodeAddr are covered by
  ethernet-only's single-root assertion and by the in-process
  spanning-tree tests respectively.
- The mixed-transport coexistence property is covered twice over by
  test_tcp_mixed_transport_coexistence and test_ble_mixed_transport, which
  call the same verify_tree_convergence_components helper with the same
  two-component shape and both run today.

So the only thing running them would have added is an AF_PACKET variant of
a property already proven on two other transports.

The counts confirm nothing running was lost: 1392 tests pass before and
after, and the ignored count falls from 7 to 4.

The ethernet-only comment block cited these tests as the reason its
assertion was the only Ethernet coverage in CI. It now records the sharper
fact found while reading them: that coverage is control plane only. Both
Ethernet scenarios disable traffic, so no datagram crosses an Ethernet
link anywhere, which leaves the frame length field that trims NIC minimum
frame padding ahead of AEAD verification unexercised. Deleting these tests
does not widen that gap, because the test named for data exchange did not
exchange any.

testing/chaos/scenarios/ethernet-only.yaml

@@ -50,10 +50,11 @@ traffic:
 # A 4-node mesh forms a spanning tree: one root and three nodes
 # with a parent. All six provably-completed archived runs show exactly
 # that, so these are the shape of a converged mesh rather than a
-# tolerance fitted to observations. This is currently the only
-# assertion covering Ethernet transport anywhere in CI: the three
-# tests in src/node/tests/ethernet.rs are #[ignore]d for requiring
-# CAP_NET_RAW and neither runner passes --ignored.
+# tolerance fitted to observations. This is the only assertion covering
+# Ethernet transport anywhere in CI, and it covers the control plane
+# only: traffic is disabled above, so no datagram crosses an Ethernet
+# link in any test. Framing, the length field that trims NIC minimum-
+# frame padding, and AEAD over Ethernet are all unexercised as a result.
 assertions:
   baseline:
     min_nodes_reporting: 4