Use shallowMount and .toHaveLength() for tests

What does this MR do?

This MR addresses the comments left by the maintainer in this MR: !29437 (merged)

It uses shallowMount instead of mount and changes all usages of .length to .toHaveLength().

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
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• [-] Separation of EE specific content