Make test data helpers more 'unique'

https://eaflood.atlassian.net/browse/WATER-4144

We've been investigating changing our unit test framework recently (see [Investigate switching to jest](#430) and [water-abstraction-ava](https://github.com/DEFRA/water-abstraction-ava)).

A key issue we'll have if we switch is that most other frameworks run tests asynchronously. This means our pattern of clean-setup-assert won't work. At best we can clean before any tests run but then the test data we setup must not clash.

To achieve that we need to make the test data the helpers generate as unique as possible, which means moving away from hard coded values.

We're still playing with frameworks but there is nothing stopping us making this change now ahead of a decision.

So, this change covers making the helpers generate more unique and fixing any tests that breaks.