API: Add tests for DateTimeUtil.microsToMillis

[vishnuprakaz]

Adds a unit test for DateTimeUtil.microsToMillis, which has had no direct
test since it was added in #2512.

The method was copied from Spark's DateTimeUtils and is used by Spark's
ProcedureInput. It uses Math.floorDiv, so pre-1970 timestamps round down
rather than toward zero the test covers a positive and a negative value to
keep that behavior correct

[vishnuprakaz]

The failing test TestStructuredStreamingRead3 > testReadingStreamFromFutureTimetsamp() (async = true) is an
unrelated flaky streaming test (timing-based, relies on a fixed
Thread.sleep). This PR only adds a unit test in the api module,
and core-tests passed. Reopening to re-trigger CI