Unflake some board specs

What does this MR do and why?

Centralise finders in one waiting finder that looks for the visible ul.board-list of the desired column.

Also ensure we assert haves before have-nots.

Part of Remove all uses of `wait_for_requests`, `wait_f... (&22879).

What even inspired this?

We have flakes showing the .board-list selection racing the v-show-hidden loading list:

• spec/features/boards/issue_ordering_spec.rb — expected to find css ".board-card" within <ul> … no matches (finder is inside the wrong, empty list):
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/16434363427 — [1:5:3] moves to index of another list, 2026-09-10
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/16428599398 — [1:3:2] moves from middle to bottom, 2026-09-10
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/16414789697 — [1:3:6] moves from bottom to middle, 2026-09-10
• spec/features/groups/board_spec.rb — undefined method base' for nil (drag_to(nil)`, target list not yet visible):
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/16434440597 — [1:2:1] allows user to move issue where they are a Reporter, 2026-09-10
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/16433995496 — same example, 2026-09-10
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/16431607226 — same example, 2026-09-10
• spec/features/groups/board_spec.rb — undefined method `all' for nil (source list not yet visible):
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/16378134045 — [1:2:2] does not allow user to move issue where they are a Guest, 2026-09-08
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/16344249943 — same, 09-07
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/16338659716 — same, 09-07

... and flakes showing spec/support/shared_examples/boards/multiple_issue_boards_shared_examples.rb's in_boards_switcher_dropdown clicking the wrapper div instead of the toggle! The dropdown never opens:

• Unable to find css ".gl-new-dropdown-item[role="option"]":
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/16434363450 — CE [1:1:1:6], 2026-09-10
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/16433058227 — CE [1:1:1:6], 2026-09-10
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/16431033844 — CE [1:1:1:7], 2026-09-10
• expected to find text "BoardX" in "BoardY" (switch never happened):
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/16360641310 — EE [1:2:1:1:2], 2026-09-08
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/16352660780 — EE [1:2:1:2:2], 2026-09-07
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/16337297944 — EE [1:2:1:1:2], 2026-09-07
• Unable to find visible button "Create new board" (via the same helper):
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/16428178949 — CE [1:1:1:4], 2026-09-10
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/16424860243 — EE [1:2:1:1:4], 2026-09-10
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/16417817902 — EE [1:2:1:1:4], 2026-09-10

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