[type:test] Improve the unit test coverage.

[yunlongn]

Improve the unit test coverage of shenyu-web, shenyu-loadbalancer.

Make sure that:

• You have read the contribution guidelines.
• You submit test cases (unit or integration tests) that back your changes.
• Your local test passed ./mvnw clean install -Dmaven.javadoc.skip=true.

[codecov-commenter]

Codecov Report

Merging #3831 (a6e7ab4) into master (6a08b6c) will increase coverage by 1.14%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #3831      +/-   ##
============================================
+ Coverage     64.74%   65.89%   +1.14%     
- Complexity     6095     6228     +133     
============================================
  Files           898      898              
  Lines         24708    24706       -2     
  Branches       2240     2240              
============================================
+ Hits          15998    16280     +282     
+ Misses         7304     7040     -264     
+ Partials       1406     1386      -20     

Impacted Files	Coverage Δ
...enyu/loadbalancer/factory/LoadBalancerFactory.java	100.00% <ø> (+33.33%)	⬆️
...che/shenyu/web/loader/ShenyuPluginPathBuilder.java	100.00% <ø> (+10.00%)	⬆️
...henyu/plugin/grpc/resolver/ShenyuNameResolver.java	62.76% <0.00%> (-1.07%)	⬇️
...enyu/client/tars/TarsServiceBeanEventListener.java	92.59% <0.00%> (+1.23%)	⬆️
...yu/web/forward/ForwardedRemoteAddressResolver.java	100.00% <0.00%> (+4.00%)	⬆️
...henyu/loadbalancer/spi/RoundRobinLoadBalancer.java	92.45% <0.00%> (+11.32%)	⬆️
...a/org/apache/shenyu/web/filter/FallbackFilter.java	100.00% <0.00%> (+14.28%)	⬆️
...java/org/apache/shenyu/web/filter/CrossFilter.java	98.41% <0.00%> (+14.28%)	⬆️
...e/shenyu/loadbalancer/cache/UpstreamCheckTask.java	93.60% <0.00%> (+15.19%)	⬆️
... and 15 more

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