Fix usage of `createdAt` and `AssignmentsStorageTestTrait`

[arogachev]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌

[what-the-diff[bot]]

PR Summary

• Assignments Timestamp Update Modified how timestamps for assignments are processed. Previously, we were using a basic time function. Now, we are getting more specific time details directly from the assignment.

• Testing Class Refinement

  • The files used in tests are updated to include necessary classes.
  • Changes made to initialize and clean up methods in the test to call the two new methods traitSetUp and traitTearDown.
  • The method that was initially used to load the database with test data was replaced by two separate methods. These allow us to fill up the items storage and assignments storage with specific test data.
  • The initial database setup for tests is currently set to be empty.
  • Lastly, two methods were included to create instances of ItemsStorage and AssignmentsStorage for testing purposes.

[samdark]

Need a line for CHANGELOG.