cmd/devp2p/ethtest: fix header stride in reverse GetHeaders

The reverse branch stepped by 1 - Skip while the forward branch steps by
1 + Skip. Skip is a uint64, so for Skip == 1 the block number never moved
and the same header was returned repeatedly, and for Skip >= 2 the
subtraction underflowed and walked forward instead of back.

The stride is Skip + 1 in both directions, so subtract 1 + Skip.

cmd/devp2p/internal/ethtest/chain.go

@@ -231,7 +231,7 @@ func (c *Chain) GetHeaders(req *eth.GetBlockHeadersPacket) ([]*types.Header, err
 	}
 	if req.Reverse {
 		for i := 1; i < int(req.Amount); i++ {
-			blockNumber -= (1 - req.Skip)
+			blockNumber -= (1 + req.Skip)
 			headers[i] = c.blocks[blockNumber].Header()
 		}
 		return headers, nil

cmd/devp2p/internal/ethtest/chain_test.go

@@ -186,6 +186,21 @@ func TestChainGetHeaders(t *testing.T) {
 				chain.Head().Header(),
 			},
 		},
+		{
+			req: eth.GetBlockHeadersPacket{
+				GetBlockHeadersRequest: &eth.GetBlockHeadersRequest{
+					Origin:  eth.HashOrNumber{Number: uint64(10)},
+					Amount:  uint64(3),
+					Skip:    1,
+					Reverse: true,
+				},
+			},
+			expected: []*types.Header{
+				chain.blocks[10].Header(),
+				chain.blocks[8].Header(),
+				chain.blocks[6].Header(),
+			},
+		},
 	}
 
 	for i, tt := range tests {