Add tests for sea-levels endpoint

https://www.pivotaltracker.com/story/show/76958360 [Delivers #76958360]
WIP Add SurgePrediction

TODO:

- add predicted_surge_level and predicted_sea_level to /sea-levels/
  endpoint
- add tests in apps.sea_levels.tests.views.test_sea_levels
- add monitoring in apps.status.views.status_index & tests

https://www.pivotaltracker.com/story/show/75795364 [#75795364]