fix: loadbalance unit test

[LaurenceLiZhixin]

What this PR does:

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-commenter]

Codecov Report

Merging #1270 (434b419) into 3.0 (968650f) will decrease coverage by 3.87%.
The diff coverage is 50.30%.

@@            Coverage Diff             @@
##              3.0    #1270      +/-   ##
==========================================
- Coverage   59.53%   55.65%   -3.88%     
==========================================
  Files         259      272      +13     
  Lines       12737    12809      +72     
==========================================
- Hits         7583     7129     -454     
- Misses       4199     4780     +581     
+ Partials      955      900      -55     

Impacted Files	Coverage Δ
cluster/cluster_impl/available_cluster_invoker.go	66.66% <ø> (ø)
cluster/cluster_impl/broadcast_cluster_invoker.go	76.47% <ø> (ø)
cluster/cluster_impl/failfast_cluster_invoker.go	66.66% <ø> (ø)
cluster/cluster_impl/mock_cluster.go	0.00% <0.00%> (ø)
cluster/loadbalance/random.go	100.00% <ø> (ø)
...router/v3router/judger/list_string_match_judger.go	0.00% <0.00%> (ø)
...ster/router/v3router/judger/method_match_judger.go	0.00% <0.00%> (ø)
...er/router/v3router/judger/url_label_match_judge.go	0.00% <0.00%> (ø)
common/extension/auth.go	0.00% <ø> (ø)
common/extension/cluster.go	0.00% <ø> (ø)
... and 284 more

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[AlexStocks]

LGTM