Find dropdown-menu before asserting active item in sidebar_spec.rb

What does this MR do?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/builds/15979237

Failures:

  1) Issue Boards assignee updates assignee dropdown
     Failure/Error: expect(page).to have_selector('.is-active')
       expected to find css ".is-active" but there were no matches
     # ./spec/features/boards/sidebar_spec.rb:167:in `block (4 levels) in <top (required)>'
     # ./spec/features/boards/sidebar_spec.rb:164:in `block (3 levels) in <top (required)>'
     # ./spec/spec_helper.rb:76:in `block (2 levels) in <top (required)>'

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?