contrib/raftexample: add test for httpKVAPI.

[johncming]

add a testcase for httpKVAPI which is an import component in raftexample.

In addition, it is helpful to solve problem like #10361.

[codecov-io]

Codecov Report

Merging #10557 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10557      +/-   ##
==========================================
+ Coverage   71.77%   71.81%   +0.04%     
==========================================
  Files         393      393              
  Lines       36573    36573              
==========================================
+ Hits        26249    26266      +17     
+ Misses       8496     8487       -9     
+ Partials     1828     1820       -8

Impacted Files	Coverage Δ
proxy/grpcproxy/watch.go	88.55% <0%> (-4.22%)	⬇️
clientv3/balancer/balancer.go	84.49% <0%> (-2.33%)	⬇️
etcdserver/api/membership/cluster.go	77.5% <0%> (-2.05%)	⬇️
pkg/adt/interval_tree.go	88.58% <0%> (-1.81%)	⬇️
pkg/netutil/netutil.go	69.67% <0%> (-0.82%)	⬇️
etcdserver/server.go	74.82% <0%> (+0.07%)	⬆️
mvcc/watchable_store.go	84.91% <0%> (+0.35%)	⬆️
etcdserver/api/rafthttp/stream.go	79.82% <0%> (+0.42%)	⬆️
clientv3/leasing/kv.go	90.36% <0%> (+0.66%)	⬆️
etcdserver/v3_server.go	79.54% <0%> (+0.68%)	⬆️
... and 10 more

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[xiang90]

lgtm