Fix tests in status module

We should always use freeze_time when testing a "disable" or "check
disabled" function in the alert manager.

Also clean up before and after each test.

https://www.pivotaltracker.com/story/show/83590686 [Delivers #83590686]
Add `is_high_tide` to TidePrediction DB model
Support is_high_tide in import_predictions command
Add `is_high_tide` to tide-levels endpoint
Add `predicted_high_tide` to sea-levels endpoint