Enchance test coverage of dubbo cluster module

[lovepoem]

What is the purpose of the change

#1679 Enhance the test coverage part-1 : dubbo-cluster module

Brief changelog

add some test case

Verifying this change

ci pass

[codecov-io]

Codecov Report

Merging #1885 into master will increase coverage by 0.34%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master   #1885      +/-   ##
===========================================
+ Coverage     48.16%   48.5%   +0.34%     
- Complexity     4347    4387      +40     
===========================================
  Files           558     558              
  Lines         24364   24362       -2     
  Branches       4323    4323              
===========================================
+ Hits          11736   11818      +82     
+ Misses        10791   10705      -86     
- Partials       1837    1839       +2

Impacted Files	Coverage Δ	Complexity Δ
...cluster/loadbalance/ConsistentHashLoadBalance.java	0% <ø> (ø)	0 <0> (ø)	⬇️
...rpc/cluster/configurator/AbstractConfigurator.java	42.55% <ø> (+1.73%)	8 <0> (ø)	⬇️
...om/alibaba/dubbo/rpc/filter/ActiveLimitFilter.java	83.33% <0%> (-5.56%)	6% <0%> (ø)
...libaba/dubbo/rpc/cluster/merger/MergerFactory.java	73.91% <0%> (ø)	6% <0%> (ø)	⬇️
...baba/dubbo/remoting/transport/mina/MinaClient.java	59.37% <0%> (+1.56%)	9% <0%> (+1%)	⬆️
...bo/remoting/transport/netty/NettyCodecAdapter.java	54.68% <0%> (+1.56%)	3% <0%> (ø)	⬇️
...rpc/protocol/dubbo/telnet/InvokeTelnetHandler.java	55.69% <0%> (+3.79%)	13% <0%> (+3%)	⬆️
...dubbo/rpc/cluster/directory/AbstractDirectory.java	62.79% <0%> (+6.97%)	12% <0%> (+2%)	⬆️
...a/dubbo/rpc/cluster/directory/StaticDirectory.java	60.71% <0%> (+32.14%)	10% <0%> (+4%)	⬆️
...om/alibaba/dubbo/rpc/cluster/merger/MapMerger.java	62.5% <0%> (+50%)	3% <0%> (+2%)	⬆️
... and 7 more

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