fix flaky test case

[ylwu-amzn]

Description of changes:

fix flaky test case

Test

./gradlew build

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[codecov]

Codecov Report

Merging #376 (dacaf2a) into master (852c2af) will decrease coverage by 0.23%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #376      +/-   ##
============================================
- Coverage     79.01%   78.77%   -0.24%     
+ Complexity     2659     2646      -13     
============================================
  Files           247      247              
  Lines         11719    11719              
  Branches       1009     1009              
============================================
- Hits           9260     9232      -28     
- Misses         1988     2005      +17     
- Partials        471      482      +11     

Flag	Coverage Δ	Complexity Δ
cli	79.27% <ø> (ø)	0.00 <ø> (ø)
plugin	78.74% <ø> (-0.26%)	0.00 <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ	Complexity Δ
...stroforelasticsearch/ad/AnomalyDetectorRunner.java	40.00% <0.00%> (-6.25%)	6.00% <0.00%> (-1.00%)
...pendistroforelasticsearch/ad/cluster/HashRing.java	78.94% <0.00%> (-5.27%)	10.00% <0.00%> (-1.00%)
...on/opendistroforelasticsearch/ad/model/ADTask.java	95.31% <0.00%> (-2.82%)	64.00% <0.00%> (-4.00%)
...roforelasticsearch/ad/task/ADTaskCacheManager.java	90.82% <0.00%> (-2.76%)	42.00% <0.00%> (-1.00%)
...asticsearch/ad/cluster/ADClusterEventListener.java	92.00% <0.00%> (-2.00%)	14.00% <0.00%> (-1.00%)
...ndistroforelasticsearch/ad/task/ADTaskManager.java	71.74% <0.00%> (-1.13%)	78.00% <0.00%> (-2.00%)
...troforelasticsearch/ad/task/ADBatchTaskRunner.java	87.82% <0.00%> (-0.87%)	63.00% <0.00%> (-3.00%)