Clean up join E2E tests (#34253)

* Add `dashboardGrid` E2E helper

* Add test ID for x-ray dashboard header

* Extract 14793 repro to its own file

* Extract 15578 repro to its own file

* Remove duplicate 17968 repro

* Clean up 17968 repro

* Clean up test

* Clean up test

* Clean up test

* Clean up test

* Clean up test

* Clean up test

* Sort imports

* Use shared helpers for join tests

* Extract 12928 repro to its own file

* Minor renames

* Fix `join` e2e helper

* Fix `assertQueryBuilderRowCount` for count > 1000

* Make table check optional in `assertJoinValid`

* Clean up test

* Fix `join` helper

* Fix 17968 test

* Fix 30058 and 32075 tests

* Reduce flakiness?