watchtower/wtmock: add in-memory impl of Queue

This commit adds an in-memory implementation of the Queue interface.
This can be used for tests.

watchtower/wtdb/queue_test.go

@@ -4,7 +4,9 @@ import (
 	"testing"
 
 	"github.com/lightningnetwork/lnd/kvdb"
+	"github.com/lightningnetwork/lnd/watchtower/wtclient"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb"
+	"github.com/lightningnetwork/lnd/watchtower/wtmock"
 	"github.com/stretchr/testify/require"
 )
 
@@ -13,21 +15,53 @@ import (
 func TestDiskQueue(t *testing.T) {
 	t.Parallel()
 
-	// Set up a temporary bolt backend.
-	dbCfg := &kvdb.BoltConfig{DBTimeout: kvdb.DefaultDBTimeout}
-	bdb, err := wtdb.NewBoltBackendCreator(
-		true, t.TempDir(), "wtclient.db",
-	)(dbCfg)
-	require.NoError(t, err)
+	dbs := []struct {
+		name string
+		init clientDBInit
+	}{
+		{
+			name: "bbolt",
+			init: func(t *testing.T) wtclient.DB {
+				dbCfg := &kvdb.BoltConfig{
+					DBTimeout: kvdb.DefaultDBTimeout,
+				}
 
-	// Construct the ClientDB.
-	db, err := wtdb.OpenClientDB(bdb)
-	require.NoError(t, err)
-	t.Cleanup(func() {
-		err = db.Close()
-		require.NoError(t, err)
-	})
+				// Construct the ClientDB.
+				bdb, err := wtdb.NewBoltBackendCreator(
+					true, t.TempDir(), "wtclient.db",
+				)(dbCfg)
+				require.NoError(t, err)
 
+				db, err := wtdb.OpenClientDB(bdb)
+				require.NoError(t, err)
+
+				t.Cleanup(func() {
+					err = db.Close()
+					require.NoError(t, err)
+				})
+
+				return db
+			},
+		},
+		{
+			name: "mock",
+			init: func(t *testing.T) wtclient.DB {
+				return wtmock.NewClientDB()
+			},
+		},
+	}
+
+	for _, database := range dbs {
+		db := database
+		t.Run(db.name, func(t *testing.T) {
+			t.Parallel()
+
+			testQueue(t, db.init(t))
+		})
+	}
+}
+
+func testQueue(t *testing.T, db wtclient.DB) {
 	namespace := []byte("test-namespace")
 	queue := db.GetDBQueue(namespace)