[WIP] test: checkpoint to a fork

This test still doesn't pass and I'm not sure why:

1. Sync is a bit broken because it can't figure out that we already have
all the data locally.
2. But with 2 nodes, it should work. Except that, if I add some logging,
I see that sync works until the libp2p nodes just flat-out disconnect
from eachother.

itests/checkpoint_test.go

@@ -0,0 +1,64 @@
+package itests
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/filecoin-project/go-state-types/abi"
+	"github.com/filecoin-project/lotus/chain/types"
+	"github.com/filecoin-project/lotus/itests/kit"
+	"github.com/stretchr/testify/require"
+)
+
+func TestCheckpointFork(t *testing.T) {
+	ctx := context.Background()
+
+	blocktime := 100 * time.Millisecond
+
+	nopts := []kit.NodeOpt{kit.WithAllSubsystems(), kit.ThroughRPC()}
+	var n1, n2 kit.TestFullNode
+	var m1, m2 kit.TestMiner
+	ens := kit.NewEnsemble(t, kit.MockProofs()).
+		FullNode(&n1, nopts...).
+		FullNode(&n2, nopts...).
+		Miner(&m1, &n1, nopts...).
+		Miner(&m2, &n1, nopts...).Start()
+
+	// Start 2 of them.
+	ens.InterconnectAll().BeginMining(blocktime)
+
+	{
+		ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+		n1.WaitTillChain(ctx, kit.HeightAtLeast(abi.ChainEpoch(5)))
+		cancel()
+	}
+
+	// Wait till both participate in a single tipset.
+	var target *types.TipSet
+	{
+		// find the first tipset where two miners mine a block.
+		ctx, cancel := context.WithTimeout(context.Background(), 5*time.Minute)
+		target = n1.WaitTillChain(ctx, func(ts *types.TipSet) bool {
+			return len(ts.Blocks()) == 2
+		})
+		cancel()
+	}
+
+	// Wait till we've moved on from that tipset.
+	targetHeight := target.Height() + 10
+	n1.WaitTillChain(ctx, kit.HeightAtLeast(targetHeight))
+	n2.WaitTillChain(ctx, kit.HeightAtLeast(targetHeight))
+
+	// Forcibly sync to this fork tipset.
+	forkTs, err := types.NewTipSet(target.Blocks()[:1])
+	require.NoError(t, err)
+	require.NoError(t, n2.SyncCheckpoint(ctx, forkTs.Key()))
+	require.NoError(t, n1.SyncCheckpoint(ctx, forkTs.Key()))
+
+	// See if we can start making progress again!
+	newHead := n1.WaitTillChain(ctx, kit.HeightAtLeast(targetHeight))
+	forkTs2, err := n1.ChainGetTipSetByHeight(ctx, forkTs.Height(), newHead.Key())
+	require.NoError(t, err)
+	require.True(t, forkTs.Equals(forkTs2))
+}