Add test asserting that feature toggling results in observable behavior change

[jaschdoc]

The test should observe that when a processor has not yet opted into enableNewFeatures, the KSPropertyDeclaration.backingField property returns null for a property declaration that has a backing field. Then it should opt in to enableNewFeatures and assert that the backingField property is the expected backing field.
The same should happen for all other change points, i.e., Resolver.getSymbolsWithAnnotation, KSPropertyDeclaration.annotations, and KSBackingField.modifiers.