[addon runmode] Fixed resolveMode in runmode.node

### Details
While using runmode.node with the "htmlmixed" mode in v3.21.0, the following error is thrown:

```
Object [object Object] has no method 'startState'
```

This is because `exports.resolveMode` is unable to correctly resolve the mode.

Modelling this fix using the getMode function in runmode.node in v3.20.0, the "spec" is correctly resolved with the appropriate mode factory function.