Add test cases for NacosMetadataReport

[laddcn]

What is the purpose of the change

Add test case for NacosMetadataReport

Brief changelog

1. Add test cases for "doSaveMetadata","removeServiceMetadata","doGetExportedURLs","doSaveSubscriberData"
2. Modify codes in NacosMetadataReportTest

Verifying this change

1. All test cases pass for NacosMetadataReportTest

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• Write necessary unit-test to verify your logic correction, more mock a little better when cross module dependency exist. If the new feature or significant change is committed, please remember to add sample in dubbo samples project.
• Run mvn clean install -DskipTests=false & mvn clean test-compile failsafe:integration-test to make sure unit-test and integration-test pass.
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[codecov-io]

Codecov Report

Merging #5776 into master will increase coverage by 0.2%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master    #5776     +/-   ##
===========================================
+ Coverage     61.17%   61.38%   +0.2%     
- Complexity      471      496     +25     
===========================================
  Files           926      979     +53     
  Lines         37843    38871   +1028     
  Branches       5461     5602    +141     
===========================================
+ Hits          23152    23862    +710     
- Misses        12179    12383    +204     
- Partials       2512     2626    +114

Impacted Files	Coverage Δ	Complexity Δ
...g/context/properties/DefaultDubboConfigBinder.java	0% <0%> (-100%)	0% <0%> (ø)
...t/annotation/DubboLifecycleComponentRegistrar.java	0% <0%> (-100%)	0% <0%> (ø)
...ache/dubbo/rpc/cluster/support/ForkingCluster.java	0% <0%> (-100%)	0% <0%> (ø)
.../context/properties/AbstractDubboConfigBinder.java	0% <0%> (-92.31%)	0% <0%> (ø)
...ring/context/config/DubboConfigBeanCustomizer.java	0% <0%> (-66.67%)	0% <0%> (ø)
...che/dubbo/rpc/cluster/support/FailfastCluster.java	50% <0%> (-50%)	0% <0%> (ø)
...ava/org/apache/dubbo/common/utils/MethodUtils.java	54.16% <0%> (-17.27%)	0% <0%> (ø)
.../org/apache/dubbo/rpc/model/ServiceRepository.java	63.63% <0%> (-14.42%)	0% <0%> (ø)
...org/apache/dubbo/common/utils/CollectionUtils.java	75.52% <0%> (-13.57%)	0% <0%> (ø)
...che/dubbo/remoting/transport/mina/MinaChannel.java	39.47% <0%> (-10.53%)	15% <0%> (-1%)
... and 148 more

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