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/graph_test.go

@@ -4128,18 +4128,10 @@ func TestGraphCacheForEachNodeChannel(t *testing.T) {
 // TestGraphLoading asserts that the cache is properly reconstructed after a
 // restart.
 func TestGraphLoading(t *testing.T) {
-	// First, create a temporary directory to be used for the duration of
-	// this test.
-	tempDirName := t.TempDir()
+	t.Parallel()
 
 	// Next, create the graph for the first time.
-	backend, backendCleanup, err := kvdb.GetTestBackend(tempDirName, "cgr")
-	require.NoError(t, err)
-	defer backend.Close()
-	defer backendCleanup()
-
-	graphStore, err := NewKVStore(backend)
-	require.NoError(t, err)
+	graphStore := NewTestDB(t)
 
 	graph, err := NewChannelGraph(graphStore)
 	require.NoError(t, err)