Opt curator-client in unit test

[kimmking]

Fixes #4773 .

Changes proposed in this pull request:

• 1、Remove testserver renewing and keep only one server during this test class running
• 2、Use final CenterRepository "client" instead of clientA， keep only one client

[codecov-io]

Codecov Report

Merging #4894 into master will decrease coverage by 0.02%.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##             master    #4894      +/-   ##
============================================
- Coverage     54.67%   54.65%   -0.03%     
+ Complexity      436      434       -2     
============================================
  Files          1138     1138              
  Lines         20412    20397      -15     
  Branches       3752     3749       -3     
============================================
- Hits          11161    11148      -13     
+ Misses         8547     8545       -2     
  Partials        704      704              

Impacted Files	Coverage Δ	Complexity Δ
...ter/instance/CuratorZookeeperCenterRepository.java	65.76% <50.00%> (-0.05%)	24.00 <1.00> (-2.00)
...rdingjdbc/jdbc/core/statement/ShadowStatement.java	62.50% <0.00%> (-1.14%)	0.00% <0.00%> (ø%)
...c/jdbc/core/statement/ShadowPreparedStatement.java	40.36% <0.00%> (-1.08%)	0.00% <0.00%> (ø%)
...dingjdbc/jdbc/core/statement/EncryptStatement.java	58.22% <0.00%> (-1.04%)	0.00% <0.00%> (ø%)
.../jdbc/core/statement/EncryptPreparedStatement.java	79.61% <0.00%> (-0.39%)	0.00% <0.00%> (ø%)
...org/apache/shardingsphere/core/rule/TableRule.java	85.26% <0.00%> (-0.16%)	1.00% <0.00%> (ø%)
.../shardingsphere/core/shard/BaseShardingEngine.java	86.95% <0.00%> (ø)	0.00% <0.00%> (ø%)
.../core/metadata/ShardingTableMetaDataDecorator.java	0.00% <0.00%> (ø)	0.00% <0.00%> (ø%)
...r/binder/metadata/column/ColumnMetaDataLoader.java	0.00% <0.00%> (ø)	0.00% <0.00%> (ø%)
...dingsphere/underlying/rewrite/SQLRewriteEntry.java	0.00% <0.00%> (ø)	0.00% <0.00%> (ø%)
... and 2 more

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[coveralls]

Pull Request Test Coverage Report for Build 10484

• 1 of 2 (50.0%) changed or added relevant lines in 1 file are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.007%) to 58.165%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
sharding-orchestration/sharding-orchestration-center/sharding-orchestration-center-zookeeper-curator/src/main/java/org/apache/shardingsphere/orchestration/center/instance/CuratorZookeeperCenterRepository.java	1	2	50.0%

Files with Coverage Reduction	New Missed Lines	%
sharding-orchestration/sharding-orchestration-core/sharding-orchestration-core-registrycenter/src/main/java/org/apache/shardingsphere/orchestration/core/registrycenter/util/IpUtils.java	3	76.0%

Totals
Change from base Build 1143:	-0.007%
Covered Lines:	11864
Relevant Lines:	20397

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💛 - Coveralls