Add end-to-end test for assignee board list

What does this MR do?

This MR adds a new end-to-end test case for the EE Premium/Silver feature: assignee board list.

Note: the new test was successfully executed on the following jobs:

• https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/422523026
• https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/422523033
• https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/422523043
• https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/422672474
• https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/422672481
• https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/422672488

Note 2: a small refactor was also made, where a private method was created to remove some code duplication (just to leave the campground a bit cleaner.)

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

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