Add unit testing for ISpellCheck

[jeanbern]

• Create with small, known corpus. Test things we know are true and things we know aren't true.
  • Use the injected string metric to validate results. Add manual results that you know were traversed in the tree (or w/e) and should not make it past the metric. Use a case where you know all the words expected, make sure they're present.
• Create with large corpus. Present with selections from corpus.