Problem/Motivation
Some tests need to submit entities with different creation date, to test sorting for example. Drupal\entity_test\Entity\EntityTestMulChanged currently facilitates this by waiting a second after each save. It works but IMHO it's suboptimal because it'll add up to a huge amount of wasted time over the years.
Proposed resolution
Change test entity definition to simulate passage of time but not actually wait idly.
Remaining tasks
Review and discuss.
User interface changes
None.
API changes
None.
Data model changes
None.