IntegrationTest is flaky for CurrentTime

[damccorm]

Currently BeamSqlDateFunctionsIntegrationTest.testDateTimeFunctions_currentTime() is a flaky test, with following error message:

• calSqlTransform/BeamCalcRel_18/ParDo(Calc)/ParMultiDo(Calc).output: null

We cannot assert the currentTime casting are correct within 1 second.

One way to solve this problem is to make a mock of system time by overriding the currentTime method and adding a way to set system time. 

Imported from Jira BEAM-10328. Original Jira may contain additional context.
Reported by: zijiesong.