Factor out rest token / user stuff to test helpers

There's some testing code in the surge endpoint which will soon be
needed in the raw measurement endpoint. Move this to the test utilities
library.
Merge pull request #110 from sealevelresearch/factor-out-test-helper

Factor out rest token / user stuff to test helpers
tide_gauge app and raw-measurements endpoint

Providing endpoints like: `/1/tide-gauges/measurements/gladstone-1/`

This is used to store raw measurements from tide gauges. These can be
processed and converted into what we currently call Observations.

https://www.pivotaltracker.com/story/show/78441914 [Delivers #78441914]