Test Fix: nil pointer error for core.NewNode

This commit fixes the errors resulting from passing a nil pointer to the
core.NewNode function in TestExternalUnmmount and setupIpnsTest.

In the previous nil's place a &core.BuildCfg{} is now passed.

Both changes follow the same pattern:

```diff
-  node, err = core.NewNode(context.Background(), nil)
+  node, err = core.NewNode(context.Background(), &core.BuildCfg{})
```

 On branch go-test-fix
 Changes to be committed:
	modified:   fuse/ipns/ipns_test.go
	modified:   fuse/node/mount_test.go
License: MIT
Signed-off-by: Chris Buesser <christopher.buesser@gmail.com>

fuse/ipns/ipns_test.go

@@ -104,7 +104,7 @@ func setupIpnsTest(t *testing.T, node *core.IpfsNode) (*core.IpfsNode, *mountWra
 
 	var err error
 	if node == nil {
-		node, err = core.NewNode(context.Background(), nil)
+		node, err = core.NewNode(context.Background(), &core.BuildCfg{})
 		if err != nil {
 			t.Fatal(err)
 		}

fuse/node/mount_test.go

@@ -42,7 +42,7 @@ func TestExternalUnmount(t *testing.T) {
 	// TODO: needed?
 	maybeSkipFuseTests(t)
 
-	node, err := core.NewNode(context.Background(), nil)
+	node, err := core.NewNode(context.Background(), &core.BuildCfg{})
 	if err != nil {
 		t.Fatal(err)
 	}