Don't sort test fixtures in place

TransactionManager's tests currently have a race in some of the test cases that share a slice of expected object IDs in their assertions. The list of expected object IDs is sorted prior to asserting for equality to ensure there are no ordering problems. As some of the tests share the same slice of expected object IDs, this leads to racy access with the sorting writing into a shared slice. Always return a new sorted slice instead to avoid this.

Closes #6071 (closed)