Field Slip system test adjustments

[nimmolo]

Refactors the field_slips_test as a sequence of actions within a single session. Should run faster and be just as effective.

Background:

With system tests, the setup/teardown for individual tests is kind of time consuming, and it seems to be more of a common pattern to test a whole “workflow of someone’s session” in a single test.

This became an issue for me recently because i’m getting intermittent failures on the Action Cable websockets branch from the field slips system test, and it seems to be coming from a rapid succession of “login” requests one right after the other.