Automated tests for issue boards

What does this MR do?

Closes gitlab-org/quality/testcases#83 (closed).

This MR adds two new automated tests.

• One to check that a board, a label board list, and an issue card are correctly created/applied through API calls
• And another one to check that a board, a milestone board list, and an issue are correctly created/applied through API calls.

Note: With the new Board, LabelBoardList, MilestoneBoardList, and ProjectMilestone resources, we will be able easily to create new tests that depend on such resources in the future.

Backport MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31903

Here is a pipeline result with the new tests passing: https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/277362468

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] 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