Extract user related itests into chef_sql_users

The itests module is too large. This patch begins the process of
extracting out related test code into smaller modules.
Move clients itests to chef_sql_clients
Extract data_bag and data_bag_item itests to separate modules
Extract nodes itests to separate module
Extract sandbox itests to separate module
Extract cookbook related itests into modules and util helper module

This one was rather tangled and so lands as a single commit. We could
likely improve the organization. But at least things are in small
enough chunks to make it easier to digest individual tests.
Cleanup application config setup in itest
Fix fetch_latest_cookbook_versions tests (they were quite broken)

First, make the tests actually run. A generator form of the assert
macro was used such that these tests were not being run.

Secondly, the helper function to generate the expected results was
returning an empty list.

Rather than mess with generating, the expected result from the specs,
simply hard-code the expected values. This makes the tests easier to
understand and keeps the implementation simple.