Correct ticking behavior with `time.time_ns()`

[jeffdn]

Previously, when freeze_time(..., tick=True) was used in conjunction with functions that called out to time.time_ns(), identical values were being returned. This was caused by the existing version of fake_time_ns rounding off the last nine digits of the returned integer value -- ensuring that each tick would be erased.

This commit only casts to int after the value has been shifted by 1e9, rather than before and after.