Re-enable `swift-xcodegen.test`

Currently failing in CI with:

```
error: keyNotFound(CodingKeys(stringValue: "output", intValue: nil), Swift.DecodingError.Context(codingPath: [_JSONKey(stringValue: "Index 0", intValue: 0)], debugDescription: "No value associated with key CodingKeys(stringValue: \"output\", intValue: nil) (\"output\").", underlyingError: nil))
```