LfsAuthTokenTest: truncate instant to millis

Java8 offers millis precision for instant however Java9+ goes further
and offers micro (6 digits).
There is no need to increase LFS auth token precision to micro
therefore truncate instant to millis for serialization and
deserialization tests.

Bug: Issue 13849
Change-Id: Icc49611879a125585385c4dc0da975871518d68a