Speed up spec/features/groups/group_runners_spec.rb

This file runs for way too long:

Finished in 3 minutes 45.6 seconds (files took 14.62 seconds to load)
54 examples, 0 failures

We can:

1. remove some tests which should be covered on other test levels
2. collapse "atomic" examples into a bigger it testing multiple things at once

From the discussion on !128607 (diffs):

This file is already way too complex and tests are very "atomic". It's a good thing most of the time, but not in the integration tests because every separate it re-runs the whole thing from scratch and waits for browser rendering.

So normally I do something like:

let!(group_runner)
let!(project_runner)
let!(instance_runner)

it 'page has everything' do
  visit_page

  expect(page).to have(group_runner)
  expect(page).to have(project_runner)
  expect(page).to have(instance_runner)
  #... 20 more expects like this
end

it 'creates a runner' do
end

it 'updates a runner' do
end

it 'deletes a runner' do
end

It saves a lot of time running tests.

See: https://about.gitlab.com/handbook/engineering/development/ops/verify/runner/#slow-rspec-test-issues