Resolve "Flaky spec in `ee/spec/features/burnup_charts_spec.rb`

What does this MR do?

Hide loading burndown/burnup charts on Iteration and Milestone pages. This fixes a bug where sometimes the charts were not visible, and a flaky test.

Screen_Recording_2021-02-09_at_11.21.18_pm

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides

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

Related to #297346 (closed)