Apply correctly the limit of 10 designs per upload

What does this MR do?

Fixes a small issue where the limit being applied to number of Designs being uploaded was 9, when the intention was 10.

Also adds some tests to cover this scenario.

Screenshots

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

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