[Dubbo-#2177] Improve unit test for AbstractRegistry team1

[Defined2014]

Improve unit test with better coverage for org.apache.dubbo.registry.support.AbstractRegistry

Run the unit test file dubbo-registry/dubbo-registry-api/src/test/java/org.apache.dubbo.registry.support/AbstractRegistryTest.java.

From Baiji, Term 269, Team 1, Issue #2177, Q2

What is the purpose of the change

Enhancement coverage for AbstractRegistry UT

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[codecov-io]

Codecov Report

Merging #2257 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #2257   +/-   ##
=========================================
  Coverage     54.57%   54.57%           
- Complexity     5156     5157    +1     
=========================================
  Files           568      568           
  Lines         25019    25019           
  Branches       4456     4456           
=========================================
  Hits          13655    13655           
+ Misses         9323     9321    -2     
- Partials       2041     2043    +2

Impacted Files	Coverage Δ	Complexity Δ
...org/apache/dubbo/rpc/filter/ActiveLimitFilter.java	83.33% <0%> (-5.56%)	6% <0%> (-1%)
...rpc/protocol/dubbo/telnet/InvokeTelnetHandler.java	51.89% <0%> (-3.8%)	11% <0%> (-2%)
.../java/org/apache/dubbo/config/ReferenceConfig.java	50.7% <0%> (-0.71%)	43% <0%> (ø)
...pache/dubbo/registry/support/AbstractRegistry.java	53.55% <0%> (+1.49%)	47% <0%> (+4%)	⬆️
...onfig/spring/extension/SpringExtensionFactory.java	85.71% <0%> (+10.71%)	10% <0%> (ø)	⬇️

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