Explicitly import gl_dropdown

Several files are missing gl_dropdown import resulting in failing several tests when being run separately^

• spec/javascripts/namespace_select_spec.js
• spec/javascripts/search_spec.js
• spec/javascripts/ci_variable_list/ci_variable_list_spec.js
• spec/javascripts/ci_variable_list/ajax_variable_list_spec.js
• spec/javascripts/new_branch_spec.js
• spec/javascripts/sidebar/sidebar_move_issue_spec.js
• spec/javascripts/issue_show/components/form_spec.js
• spec/javascripts/issue_show/components/fields/description_template_spec.js
• spec/javascripts/create_item_dropdown_spec.js
• spec/javascripts/todos_spec.js

See #67206 (moved) for details

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content