Attempt to fix flaky test in `user_views_iteration_spec.rb`

What does this MR do?

This MR attempts to fix the tests quarantined in !56187 (merged).

The iteration report group by label shared example does the following UI actions:

Screen_Recording_2021-03-23_at_1.54.10_pm

• click_button 'Label' clicks on the Label dropdown, which is followed by a REST API call
• click_link label1.title clicks on a label in the dropdown
• send_keys(:escape) closes the dropdown, which is followed by a GraphQL API call

This MR adds wait_for_requests to wait for the REST and GraphQL API calls.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• 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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #323731 (closed)