Add dictionary vector unit tests

[TheNeuralBit]

Adds some tests of Dictionary Vectors both with nullable and non-nullable index vectors. The nullable tests fail on master, but pass on this branch. Except for slicing a DictionaryVector with nullCount > 0, which fails with the following error:

  ● DictionaryVector › index with nullCount > 0 › sliced › encountered a declaration exception

    TypeError: Unrecognized DictionaryVector view

      495 |         }
      496 |         else {
    > 497 |             throw new TypeError("Unrecognized DictionaryVector view");
      498 |         }
      499 |         return _this;
      500 |     }
      
      at new DictionaryVector (targets/es5/cjs/vector.js:497:19)
      at DictionaryVector.Object.<anonymous>.Vector.clone (targets/es5/cjs/vector.js:70:16)
      at DictionaryVector.Object.<anonymous>.Vector.slice (targets/es5/cjs/vector.js:122:21)
      at Suite.describe (test/unit/vector-tests.ts:350:37)
      at Suite.describe (test/unit/vector-tests.ts:349:9)
      at Suite.Object.<anonymous>.describe (test/unit/vector-tests.ts:342:5)
      at Object.<anonymous> (test/unit/vector-tests.ts:320:1)

[TheNeuralBit]

Think I found the problem. The Vector constructor wraps the DictionaryView in a ValidityView here, but there's no logic to handle it in the DictionaryVector constructor, which gets called as part of a slice.