[hotfix] Add sleep to stop intermittent failure in ExecutionTimeBasedSlowTaskDetectorTest

[davidradl]

We were getting an intermittent error in our CI pipeline, as below. This change adds a small sleep to allow the 2 slow tasks to start before it checks they are there. This approach is in line with other parts of this code e.g. this

[ERROR] Tests run: 13, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 2.068 s <<< FAILURE! -- in org.apache.flink.runtime.scheduler.slowtaskdetector.ExecutionTimeBasedSlowTaskDetectorTest
#24 511.0 [ERROR] org.apache.flink.runtime.scheduler.slowtaskdetector.ExecutionTimeBasedSlowTaskDetectorTest.testBalancedInput -- Time elapsed: 0.011 s <<< FAILURE!
#24 511.0 java.lang.AssertionError:
#24 511.0
#24 511.0 Expected size: 2 but was: 0 in:
#24 511.0 {}
#24 511.0 at org.apache.flink.runtime.scheduler.slowtaskdetector.ExecutionTimeBasedSlowTaskDetectorTest.testBalancedInput(ExecutionTimeBasedSlowTaskDetectorTest.java:269)
#24 511.0 at java.base/java.lang.reflect.Method.invoke(Method.java:586)
#24 511.0 at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:387)
#24 511.0 at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1312)
#24 511.0 at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1843)
#24 511.0 at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1808)
#24 511.0 at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:188)

[flinkbot]

CI report:

• 9e8463c Azure: SUCCESS

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