cmd/devp2p/internal/ethtest: return request ID in BlockHeaders response (ethereum#22508)

This PR fixes an issue with the eth66 test suite where, during a readAndServe when
the test is manually responding to GetBlockHeader requests, it now responds
with a BlockHeaders eth66 packet that includes the inbound request ID.

Author: renaynay

cmd/devp2p/internal/ethtest/eth66_suiteHelpers.go

@@ -141,8 +141,11 @@ func (c *Conn) readAndServe66(chain *Chain, timeout time.Duration) (uint64, Mess
 			if err != nil {
 				return 0, errorf("could not get headers for inbound header request: %v", err)
 			}
-
-			if err := c.Write(headers); err != nil {
+			resp := &eth.BlockHeadersPacket66{
+				RequestId:          reqID,
+				BlockHeadersPacket: eth.BlockHeadersPacket(headers),
+			}
+			if err := c.write66(resp, BlockHeaders{}.Code()); err != nil {
 				return 0, errorf("could not write to connection: %v", err)
 			}
 		default: