Change let_it_be to let! in multiple-epic-board-spec

What does this MR do and why?

Replace let_it_be with let! in ee/spec/features/epic_boards/multiple_epic_boards_spec.rb to resolve flaky specs reported in:

• Flaky test: ee/spec/features/epic_boards/multip... (#390744 - closed)
• Flaky test: ee/spec/features/epic_boards/multip... (#396791 - closed)

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