channeldb: remove graph db from channeldb

Now that the channel.DB no longer uses the ChannelGraph pointer, we can
completely remove it as a member of channeldb.DB.

graph/db/graph.go

@@ -12,6 +12,7 @@ import (
 	"net"
 	"sort"
 	"sync"
+	"testing"
 	"time"
 
 	"github.com/btcsuite/btcd/btcec/v2"
@@ -4775,3 +4776,31 @@ func deserializeChanEdgePolicyRaw(r io.Reader) (*models.ChannelEdgePolicy,
 
 	return edge, nil
 }
+
+// MakeTestGraph creates a new instance of the ChannelGraph for testing purposes.
+func MakeTestGraph(t testing.TB, modifiers ...OptionModifier) (*ChannelGraph, error) {
+	opts := DefaultOptions()
+	for _, modifier := range modifiers {
+		modifier(opts)
+	}
+
+	// Next, create channelgraph for the first time.
+	backend, backendCleanup, err := kvdb.GetTestBackend(t.TempDir(), "cgr")
+	if err != nil {
+		backendCleanup()
+		return nil, err
+	}
+
+	graph, err := NewChannelGraph(backend)
+	if err != nil {
+		backendCleanup()
+		return nil, err
+	}
+
+	t.Cleanup(func() {
+		_ = backend.Close()
+		backendCleanup()
+	})
+
+	return graph, nil
+}