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>