Add test for exists behaviour on lists of direct relations

[erlendvollset]

Description

Please describe the change you have made.

Checklist:

• [ ] Tests added/updated.
• [ ] Documentation updated. Documentation is generated from docstrings - these must be updated according to your change. If a new method has been added it should be referenced in cognite.rst in order to generate docs based on its docstring.
• [ ] Changelog updated in CHANGELOG.md.
• [ ] Version bumped. If triggering a new release is desired, bump the version number in _version.py and pyproject.toml per semantic versioning.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.27%. Comparing base (0941557) to head (9be41a6).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2005 +/- ## ======================================= Coverage 90.27% 90.27% ======================================= Files 139 139 Lines 21882 21882 ======================================= Hits 19753 19753 Misses 2129 2129 ``` | [Files with missing lines](https://app.codecov.io/gh/cognitedata/cognite-sdk-python/pull/2005?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cognitedata) | Coverage Δ | | |---|---|---| | [cognite/client/\_api/data\_modeling/instances.py](https://app.codecov.io/gh/cognitedata/cognite-sdk-python/pull/2005?src=pr&el=tree&filepath=cognite%2Fclient%2F_api%2Fdata_modeling%2Finstances.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cognitedata#diff-Y29nbml0ZS9jbGllbnQvX2FwaS9kYXRhX21vZGVsaW5nL2luc3RhbmNlcy5weQ==) | `88.20% <100.00%> (ø)` | | | [...nite/client/data\_classes/data\_modeling/\_\_init\_\_.py](https://app.codecov.io/gh/cognitedata/cognite-sdk-python/pull/2005?src=pr&el=tree&filepath=cognite%2Fclient%2Fdata_classes%2Fdata_modeling%2F__init__.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cognitedata#diff-Y29nbml0ZS9jbGllbnQvZGF0YV9jbGFzc2VzL2RhdGFfbW9kZWxpbmcvX19pbml0X18ucHk=) | `100.00% <ø> (ø)` | |