Cleanup query tests to avoid error

[ricellis]

Bug Description

1. Steps to reproduce and the simplest code sample possible to demonstrate the issue

Run the CDTDatastoreQuerySpec tests in release mode. Since the CI runs in release mode now to try and avoid issues like #435 these tests will need fixing up.

2. What you expected to happen

Tests to run to completion without libsqlite3.dylib errors.

3. What actually happened

Test Suite 'CDTDatastoreQuerySpec' started at 2018-07-09 16:07:07.415
Test Case '-[CDTDatastoreQuerySpec test_When_using_datastore_query__creates_different_index_managers_for_different_datastores]' started.
2018-07-09 16:07:07.635221+0100 xctest[17344:6757974] [logging] BUG IN CLIENT OF libsqlite3.dylib: database integrity compromised by API violation: vnode unlinked while in use: /private/var/folders/w8/nl63tml92z99wgzjlf109b400000gn/T/cloudant_sync_ios_tests.KfXYcd/test2_extensions/com.cloudant.sync.query/indexes.sqlite

• 11 more instances.

Environment details

• Version(s) that are affected by this issue.

  fix-cdtq-weak-reference branch

• iOS/macOS version

  macOS 10.13.5