functional-tester: run network fault tests with snapshot trigger

[gyuho]

And some clean ups.

And move proxy to a separate package (ref. #9081).

[codecov-io]

Codecov Report

Merging #9530 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9530      +/-   ##
==========================================
+ Coverage   72.41%   72.44%   +0.02%     
==========================================
  Files         367      367              
  Lines       31267    31267              
==========================================
+ Hits        22643    22652       +9     
+ Misses       6992     6977      -15     
- Partials     1632     1638       +6

Impacted Files	Coverage Δ
compactor/periodic.go	56.06% <0%> (-33.34%)	⬇️
proxy/grpcproxy/register.go	69.44% <0%> (-11.12%)	⬇️
etcdserver/api/v3rpc/lease.go	74.32% <0%> (-6.76%)	⬇️
auth/simple_token.go	87.03% <0%> (-6.49%)	⬇️
proxy/grpcproxy/watcher.go	89.79% <0%> (-4.09%)	⬇️
etcdserver/api/v3election/election.go	66.66% <0%> (-2.78%)	⬇️
clientv3/concurrency/election.go	79.52% <0%> (-2.37%)	⬇️
lease/leasehttp/http.go	58.08% <0%> (-1.48%)	⬇️
etcdserver/cluster_util.go	76.64% <0%> (-1.46%)	⬇️
clientv3/balancer/grpc1.7-health.go	85.17% <0%> (-1.17%)	⬇️
... and 19 more

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