simulator: Fix timestamps manipulation

It's unclear whether the change in this mr triggers the bug in the simulator.
But it should be timestamp that needs manipulation and it's currently
datetimes. So this makes the current state fails with [1]

Adding the conversion layer from datetime to timestamps make the tests happier.

[1]
```
16:17:06  low = datetime.datetime(2025, 3, 14, 15, 17, 2, 139077, tzinfo=datetime.timezone.utc)
16:17:06  high = datetime.datetime(2025, 3, 14, 15, 17, 2, 139077, tzinfo=datetime.timezone.utc)
16:17:06
16:17:06      def _diff(low, high):
16:17:06          if low == high:
16:17:06              if low == 0:
16:17:06                  return 0.5
16:17:06              else:
16:17:06  >               return abs(low * 0.1)
16:17:06  E               TypeError: unsupported operand type(s) for *: 'datetime.datetime' and 'float'
16:17:06
```

Refs. swh/infra/sysadm-environment#5512