Improve code coverage of nacos config center and configuration parser

[Patrick0308]

Improve code coverage of nacos config center and configuration parser

relates #518

[codecov-commenter]

Codecov Report

Merging #587 into develop will increase coverage by 0.37%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #587      +/-   ##
===========================================
+ Coverage    66.80%   67.18%   +0.37%     
===========================================
  Files          185      188       +3     
  Lines         9749     9849     +100     
===========================================
+ Hits          6513     6617     +104     
+ Misses        2600     2581      -19     
- Partials       636      651      +15     

Impacted Files	Coverage Δ
cluster/cluster_impl/base_cluster_invoker.go	62.31% <0.00%> (-10.15%)	⬇️
protocol/dubbo/listener.go	56.45% <0.00%> (-6.46%)	⬇️
protocol/dubbo/pool.go	75.00% <0.00%> (-6.37%)	⬇️
protocol/dubbo/client.go	64.24% <0.00%> (-4.85%)	⬇️
config_center/dynamic_configuration.go	62.50% <0.00%> (ø)
config_center/mock_dynamic_config.go	0.00% <0.00%> (ø)
config_center/configuration_listener.go	0.00% <0.00%> (ø)
registry/kubernetes/registry.go	67.30% <0.00%> (+9.61%)	⬆️
config_center/nacos/client.go	75.21% <0.00%> (+19.65%)	⬆️
config_center/nacos/factory.go	62.50% <0.00%> (+50.00%)	⬆️
... and 2 more

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