Move dashboard dropdown specs to dashboard_header_spec.js

What does this MR do?

Move dashboard dropdown specs to the right file

After creating the dashboard header, the specs where not moved, this MR addresses moving part of them by moving the dashboard dropdown specs.

Screenshots

N/A

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done