JS tests that use dbHash incorrectly check number of collections

max.hirschhorn noticed that the fix for SERVER-21768 incorrectly checks .length fields on objects. For example, in this change, .length is now being called on a 'collections' object.

assert.eq(masterHashes.collections.length,

-                              slaveHashes.numCollections,

+                              slaveHashes.collections.length,

We should fix the tests to compare Object.keys() counts instead.