eth/tracers: fix test (ethereum#31445)

This pull request fixes a broken unit test

```
=== CONT  TestTracingWithOverrides
    api_test.go:1012: result: {"gas":21167,"failed":false,"returnValue":"0x0000000000000000000000000000000000000000000000000000000000000002","structLogs":[{"pc":0,"op":"PUSH1","gas":24978860,"gasCost":3,"depth":1,"stack":[]},{"pc":2,"op":"CALLDATALOAD","gas":24978857,"gasCost":3,"depth":1,"stack":["0x0"]},{"pc":3,"op":"PUSH1","gas":24978854,"gasCost":3,"depth":1,"stack":["0x1"]},{"pc":5,"op":"ADD","gas":24978851,"gasCost":3,"depth":1,"stack":["0x1","0x1"]},{"pc":6,"op":"PUSH1","gas":24978848,"gasCost":3,"depth":1,"stack":["0x2"]},{"pc":8,"op":"MSTORE","gas":24978845,"gasCost":6,"depth":1,"stack":["0x2","0x0"]},{"pc":9,"op":"PUSH1","gas":24978839,"gasCost":3,"depth":1,"stack":[]},{"pc":11,"op":"PUSH1","gas":24978836,"gasCost":3,"depth":1,"stack":["0x20"]},{"pc":13,"op":"RETURN","gas":24978833,"gasCost":0,"depth":1,"stack":["0x20","0x0"]}]}
    api_test.go:1013: test 10, result mismatch, have
        {21167 false 0x0000000000000000000000000000000000000000000000000000000000000002}
        , want
        {21167 false 0000000000000000000000000000000000000000000000000000000000000002}
    api_test.go:1012: result: {"gas":25664,"failed":false,"returnValue":"0x000000000000000000000000c6e93f4c1920eaeaa1e699f76a7a8c18e3056074","structLogs":[]}
    api_test.go:1013: test 11, result mismatch, have
        {25664 false 0x000000000000000000000000c6e93f4c1920eaeaa1e699f76a7a8c18e3056074}
        , want
        {25664 false 000000000000000000000000c6e93f4c1920eaeaa1e699f76a7a8c18e3056074}

```

Author: rjl493456442

eth/tracers/api_test.go

@@ -960,7 +960,7 @@ func TestTracingWithOverrides(t *testing.T) {
 					},
 				},
 			},
-			want: `{"gas":21167,"failed":false,"returnValue":"0000000000000000000000000000000000000000000000000000000000000002"}`,
+			want: `{"gas":21167,"failed":false,"returnValue":"0x0000000000000000000000000000000000000000000000000000000000000002"}`,
 		},
 		{ // Call to ECREC Precompiled on a different address, expect the original behaviour of ECREC precompile
 			blockNumber: rpc.LatestBlockNumber,
@@ -981,7 +981,7 @@ func TestTracingWithOverrides(t *testing.T) {
 					},
 				},
 			},
-			want: `{"gas":25664,"failed":false,"returnValue":"000000000000000000000000c6e93f4c1920eaeaa1e699f76a7a8c18e3056074"}`,
+			want: `{"gas":25664,"failed":false,"returnValue":"0x000000000000000000000000c6e93f4c1920eaeaa1e699f76a7a8c18e3056074"}`,
 		},
 	}
 	for i, tc := range testSuite {