fix unit test

[vdiskg]

newSingleThreadExecutor has a bug, and it only fixed on jdk11. while running on jdk8, it may break the unit test by accident (depend on gc)
https://bugs.openjdk.java.net/browse/JDK-8145304

[codecov-commenter]

Codecov Report

Merging #3669 (821308e) into master (20b8ddb) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #3669   +/-   ##
=========================================
  Coverage     50.98%   50.98%           
+ Complexity     2331     2330    -1     
=========================================
  Files           449      449           
  Lines         14026    14026           
  Branches       1426     1426           
=========================================
  Hits           7151     7151           
+ Misses         6390     6389    -1     
- Partials        485      486    +1     

Impacted Files	Coverage Δ	Complexity Δ
.../apollo/internals/RemoteConfigLongPollService.java	77.10% <0.00%> (-1.21%)	28.00% <0.00%> (-1.00%)
...lo/configservice/util/InstanceConfigAuditUtil.java	76.06% <0.00%> (+1.70%)	15.00% <0.00%> (ø%)

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[nobodyiam]

LGTM