testutilties: New NanosecondsSinceEpoch function

Extends the test utilities by the new time-since-epoch function
`NanosecondsSinceEpoch()` that retrieves the time since epoch in
nanoseconds and not in seconds (like `SecondsSinceEpoch()`).
testutilities: Randomizing functions use NanosecondsSinceEpoch()

Change the seed for the randomizer functions to `NanosecondsSinceEpoch()`
instead of `SecondsSinceEpoch()`. The new function has a finer time
grading, so the seed becomes more random.