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/kv_store.go

@@ -4709,6 +4709,9 @@ func (c *chanGraphNodeTx) ForEachChannel(f func(*models.ChannelEdgeInfo,
 
 // MakeTestGraph creates a new instance of the ChannelGraph for testing
 // purposes.
+//
+// NOTE: this helper currently creates a ChannelGraph that is only ever backed
+// by the `KVStore` of the `V1Store` interface.
 func MakeTestGraph(t testing.TB, opts ...ChanGraphOption) *ChannelGraph {
 	t.Helper()