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.

[coveralls]

coverage: 94.45%. remained the same
when pulling d140e92 on nimmo-system-test-class-selector-adjustments
into 4f69b78 on main.

[nimmolo]

Update: #2107 passes now, so i don't consider this necessary.
It does however have some things we may want to keep:

• Adds identifying selector classes to the important links on Field Slips, so the system test never has to use xpath. xpath selectors tend to be brittle if HTML is reorganized.
• The test runs 30-50% faster (3-5s for me locally, and similar in CI) when all strung together, no matter how many times I re-run it. It's a small gain here, but i think does illustrate that there's a material difference from consolidating a system test in a workflow.
• Parens for method args is MO's Rubocop style across the whole app. I feel like if we're going to have a style...