test: protobuf vectors for reactors (#5221)

## Description

Add test vectors for all reactors

- [x] state-sync
- [x] privval
- [x] mempool
- [x] p2p
- [x] evidence
- [ ] light?

this PR is primarily oriented at testvectors for things going over the wire. should we expand the testvectors into types as well?

Closes: #XXX

mempool/reactor_test.go

@@ -1,6 +1,7 @@
 package mempool
 
 import (
+	"encoding/hex"
 	"errors"
 	"net"
 	"sync"
@@ -10,12 +11,14 @@ import (
 	"github.com/fortytw2/leaktest"
 	"github.com/go-kit/kit/log/term"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 
 	"github.com/tendermint/tendermint/abci/example/kvstore"
 	cfg "github.com/tendermint/tendermint/config"
 	"github.com/tendermint/tendermint/libs/log"
 	"github.com/tendermint/tendermint/p2p"
 	"github.com/tendermint/tendermint/p2p/mock"
+	memproto "github.com/tendermint/tendermint/proto/tendermint/mempool"
 	"github.com/tendermint/tendermint/proxy"
 	"github.com/tendermint/tendermint/types"
 )
@@ -246,3 +249,30 @@ func ensureNoTxs(t *testing.T, reactor *Reactor, timeout time.Duration) {
 	time.Sleep(timeout) // wait for the txs in all mempools
 	assert.Zero(t, reactor.mempool.Size())
 }
+
+func TestMempoolVectors(t *testing.T) {
+
+	testCases := []struct {
+		testName string
+		tx       []byte
+		expBytes string
+	}{
+		{"tx 1", []byte{123}, "0a030a017b"},
+		{"tx 2", []byte("proto encoding in mempool"), "0a1b0a1970726f746f20656e636f64696e6720696e206d656d706f6f6c"},
+	}
+
+	for _, tc := range testCases {
+		tc := tc
+
+		msg := memproto.Message{
+			Sum: &memproto.Message_Tx{
+				Tx: &memproto.Tx{Tx: tc.tx},
+			},
+		}
+		bz, err := msg.Marshal()
+		require.NoError(t, err, tc.testName)
+
+		require.Equal(t, tc.expBytes, hex.EncodeToString(bz), tc.testName)
+	}
+
+}