Add tests for fieldable content entities with non-integer IDs

Thank you for creating this issue to improve Drupal.

We are working to decide if this task is still relevant to a currently supported version of Drupal. There hasn't been any discussion here for over 8 years which suggests that this has either been implemented or is no longer relevant. Your thoughts on this will allow a decision to be made.

Since we need more information to move forward with this issue, the status is now Postponed (maintainer needs more info). If we don't receive additional information to help with the issue, it may be closed after three months.

Thanks!

We have several test entity types with string ids now, there are likely gaps in the test coverage for them, but I don't think still having this issue open will help with closing those, so lets close it.