Unskip and improve specs marked as flaky in system/volunteers/index_spec

[solebared]

What github issue is this PR for, if any?

Hopefully fixes #5727

What changed, and why?

1. Uncommented and unskipped a bunch of examples in spec/system/volunteers/index_spec.rb

2. Eliminated all wait:s to try to recreate flakes in CI, but didn't run into any.

3. Converted a bunch of find-then-assert occurrences to use corresponding capybara matchers instead. Eg:

   -  expect(page.all("table#volunteers tbody tr").count).to eq inactive_volunteers.count
   +  expect(page).to have_selector("table#volunteers tbody tr", count: inactive_volunteers.count)Logo']")

4. Hoping we can merge this in and see if we run into any flakiness over the weekend.

Feelings gif (optional)

[schoork]

Woot woot!