mongodriver follow up: re-enable disabled unit tests

[fgalan]

During the work on issue #3132 (PR #3622), the unit tests for mongoBackend were disabled, in order to fully remove the dependencies with the old legacy driver along the code base.

This is not a big lost (unit tests were very useful at the beginning of Context Broker development, to fully test big modules before "assembly" them together) but since years ago functional tests are the important ones.

Having said that, it would be great if we could recover disabled tests. In two stages:

1. Recover old the tests cases not needed mongo mocks
2. Fix the current mocks and enable the pending tests which need such mocks

[fgalan]

Look for ocurrences of FIXME #3775 in the code about this issue.