Tests

- unit tests for Observer:
  - unobserve()
  - ensurePath()
  - ensurePaths()
  - Multiple observers
- unit test for sd-model:
  - should only lock update if it has no filters
- functional test for sd-model in validation.js:
  - should remember and restore selection position
    when it has filters
- functional test for Observer.ensurePaths():
  - should allow setting a nested scope object to {}