Fix FeverAPI test flakiness

Attempts to address #331, mainly by stubbing `Time.now`.

* Adds whitespace, splitting tests into setup, execution and verification steps.

* Fixes a typo (`standart` -> `standard`).

* Introduces `last_response_as_object` helper.

  This allows splitting the assertions into two parts, one asserting inclusion of standard keys and one asserting inclusion of keys specific to each action.

* Stubs `Time.now`, splits assertions and makes use of the includes matcher.
Although I think an integer would be sufficient for our purposes, I can see your point. I'll fix this. Thanks for your observation!