Add assign project milestone spec

What does this MR do?

Adds a test covering assigning project milestones through the UI.

Covers test gaps mentioned here gitlab-org/quality/testcases#830 (closed)

Screenshots

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