Previously, test code that generated random durations could create a
duration with negative amount of nanoseconds. Such duration is now
normalized by the database so that nanoseconds value is always in
range from 0 to 999_999_999.
This commit makes tests only create random durations with nanoseconds
in that range. Otherwise some test assertions fail because they expect
non-normalized value to be returned back.

Database normalizes nanoseconds to be in range from 0 to 999_999_999.
`IsoDuration` previously did no calculations in it's `#toString()`
and returned incorrect result.
This commit makes it return correct string representation despite
normalized second and nanosecond components. It will also do
normalization in the constructor using
`java.time.Duration#ofSeconds(long, long)` in order to return correct
strings for durations created on the client side and not returned
from the database.