graph/db: add NewTestDB method

In this commit, we add a `test_kvdb.go` file with a single definition of
the `NewTestDB` function. A new version of `MakeTestGraph` (called
`MakeTestGraphNew` is added which makes use of this `NewTestDB` function
to create the backing `V1Store` passed to the `ChannelGraph` for tests.

Later on, we will add new implementations of this method backed by
sqlite and postgres.  When those are added, then build flags will
control which version of `NewTestDB` is called.

With this change, the only test call-site of `NewKVStore` is the new
`test_kvdb.go` file.

graph/db/test_kvdb.go

@@ -0,0 +1,21 @@
+package graphdb
+
+import (
+	"testing"
+
+	"github.com/lightningnetwork/lnd/kvdb"
+	"github.com/stretchr/testify/require"
+)
+
+// NewTestDB is a helper function that creates an BBolt database for testing.
+func NewTestDB(t testing.TB) *KVStore {
+	backend, backendCleanup, err := kvdb.GetTestBackend(t.TempDir(), "cgr")
+	require.NoError(t, err)
+
+	t.Cleanup(backendCleanup)
+
+	graphStore, err := NewKVStore(backend)
+	require.NoError(t, err)
+
+	return graphStore
+}