Add test for FileIO mode

### 📝 Description of the feature

Adding this as a reminder to add a test for FileIO mode once we are using the 261 container.

The changes that had had to be made to support FileIO mode were mainly refactoring so that it was possible to switch from the existing Cosimulation mode. Most of the code changes were exercised by existing tests which showed that Cosimulation mode continued to work.

The new code that is exercised only for the FileIO case is minimal so coverage was barely affected, but it would be good to demonstrate FileIO mode working in a test.

### 💡 Steps for implementing the feature

To be determined, but can be quite basic. Just needs to demonstrate starting SyC in FileIO mode and making an API call that is only available in that mode.

### 🔗 Useful links and references

_No response_

Start by locating the existing Cosimulation tests and the configuration for the 261 container, then find the FileIO setup and its mode-specific API. Done means a test starts SyC in FileIO mode and makes an API call available only in that mode.

Written by the indexing model from the issue text.