* Add setting to enable testing mode which is used in engine-gecko to set `FULL_ACCESSIBILITY_TREE` to `true`. This allows access to the full DOM tree for testing purposes.
```Kotlin
// Turn testing mode on by default when the engine is created
val engine = GeckoEngine(runtime, DefaultSettings(testingModeEnabled=true))
// Or turn testing mode on at a later point
engine.settings.testingModeEnabled = true
```
* The existing `userAgentString` setting is now supported by `engine-gecko-nightly`.