test: use unique child names when creating devices with an explicit @id field

[reubenmiller]

Proposed changes

Use unique names within the system test which registers devices using an explicit @id field to avoid name clashes when running multiple tests or if the device is not cleaned up in Cumulocity IoT (as the @id is used as the External Id which must be unique within the whole tenant, and can only have 1 owner).

This is the same issue fixed in other tests in #2618

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (general improvements like code refactoring that doesn't explicitly fix a bug or add any new functionality)
• Documentation Update (if none of the other choices apply)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Paste Link to the issue

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA (in all commits with git commit -s)
• I ran cargo fmt as mentioned in CODING_GUIDELINES
• I used cargo clippy as mentioned in CODING_GUIDELINES
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

[github-actions]

Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
389	0	3	389	100	50m12.272s