This commit adds a method to the integration tester that allows
comparing two DateTimeInterface objects and specify tolerated delta.
It also allows passing null and assert the DateTimeInterface internally.
This is because often our entities have return type DateTimeInterface|null
So with the internal checks we don't have to make those instance of checks
each time we pass date from entity into the method.
Note: I was not able to use $this->assertInstanceOf because PHPStan was not accepting that
and still complained.
[MAILPOET-4723]
