nexmark.sources.UnboundedEventSourceTest.resumeFromCheckpoint is flaky (sickbayed)

This test fails fairly frequently.

History: [https://builds.apache.org/view/A-D/view/Beam/job/beam_PostCommit_Java_GradleBuild/1219/testReport/junit/org.apache.beam.sdk.nexmark.sources/UnboundedEventSourceTest/resumeFromCheckpoint/history/](https://builds.apache.org/view/A-D/view/Beam/job/beam_PostCommit_Java_GradleBuild/1219/testReport/junit/org.apache.beam.sdk.nexmark.sources/UnboundedEventSourceTest/resumeFromCheckpoint/history/)

 Sample job: https://builds.apache.org/view/A-D/view/Beam/job/beam_PostCommit_Java_GradleBuild/1219/testReport/org.apache.beam.sdk.nexmark.sources/UnboundedEventSourceTest/resumeFromCheckpoint/

Failure log:

org.junit.ComparisonFailure: expected:<...":"UTC"},"afterNow":[true,"beforeNow":fals]e,"equalNow":false},...\> but was:<...":"UTC"},"afterNow":[false,"beforeNow":tru]e,"equalNow":false},...\> at org.junit.Assert.assertEquals(Assert.java:115) at org.junit.Assert.assertEquals(Assert.java:144) at org.apache.beam.sdk.nexmark.sources.UnboundedEventSourceTest$EventIdChecker.add(UnboundedEventSourceTest.java:71) at org.apache.beam.sdk.nexmark.sources.UnboundedEventSourceTest.resumeFromCheckpoint(UnboundedEventSourceTest.java:96)

Imported from Jira [BEAM-5070](https://issues.apache.org/jira/browse/BEAM-5070). Original Jira may contain additional context.
Reported by: reuvenlax.