test: Prevent conflict in test fixtures ids

[MonkeyDo]

Problem

Tests were failing erratically because of two reasons:

1. We were assigning an id to pretty much all the test fixtures we created using the ORM, when it's not necessary
2. The random number generator we were using to generate the ids was creating conflicting ids

Solution

Let the database create ids, which required a few changes in the test helpers.

Areas of Impact

Tests only.

[MonkeyDo]

@prabalsingh24 Requesting your eyes to double-check in case you spot something stupid :)

[coveralls]

Coverage increased (+0.08%) to 62.721% when pulling bd77e6d on test-fixtures-ids into fae8612 on master.

[MonkeyDo]

I'll merge this now, but you can still review and comment if you spot anything !