Simplify flaky runner pause/resume spec assertions

What does this MR do and why?

The 'pauses and resumes runner' shared example has been a recurring flaky test. Previous attempts (!240917 (merged), !242256 (merged)) tried to stabilize assertions on the toggle button's label and disabled state — for example have_button 'Resume', disabled: false and not_to have_button 'Pause'.

Those assertions race the in-flight runnerUpdate GraphQL mutation: the button passes through a transitional loading/disabled state before the component re-renders, so even the disabled: false waits did not fully eliminate the flake (still failing on master, see the referenced issue).

This MR drops the button-state assertions entirely and asserts only on the "Paused" status text, which is the stable end-state of the pause and resume mutations:

• before pausing: Paused text is absent
• after clicking Pause: Paused text is present
• after clicking Resume: Paused text is absent again

This is a simpler assertion that no longer depends on transient button states, while still exercising the same pause/resume logic end-to-end.

References

• https://gitlab.com/gitlab-org/quality/test-failure-issues/-/work_items/43165

How to verify

bin/rspec spec/features/groups/runners/owner_manages_runners_spec.rb
bin/rspec spec/features/projects/runners/user_edits_runner_spec.rb