test(loadbalance): add unittests for p2c

[justxuewei]

What this PR does:

• The unit test coverage in the p2c load balance is up to 82.6%.
• Remove protocol.NewRPCResult from the code.

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 #1686 (f1bf1cb) into 3.0 (0e195d1) will increase coverage by 0.17%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##              3.0    #1686      +/-   ##
==========================================
+ Coverage   41.22%   41.40%   +0.17%     
==========================================
  Files         254      255       +1     
  Lines       14614    14664      +50     
==========================================
+ Hits         6025     6072      +47     
- Misses       7893     7894       +1     
- Partials      696      698       +2     

Impacted Files	Coverage Δ
protocol/invocation/rpcinvocation.go	15.83% <ø> (ø)
protocol/result.go	0.00% <ø> (ø)
cluster/cluster/available/cluster_invoker.go	73.33% <33.33%> (ø)
cluster/loadbalance/p2c/loadbalance.go	76.36% <72.72%> (ø)
filter/sentinel/filter.go	52.45% <100.00%> (ø)
metrics/prometheus/reporter.go	28.57% <0.00%> (+1.71%)	⬆️
...tocol/rest/server/server_impl/go_restful_server.go	44.82% <0.00%> (+3.44%)	⬆️

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[Lvnszn]

LGTM