e2e: improve Member TLS test coverage.

[hexfusion]

Part of ongoing e2e coverage improvements.

ref: #9149

[codecov-io]

Codecov Report

Merging #9184 into master will decrease coverage by 0.24%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9184      +/-   ##
==========================================
- Coverage   76.18%   75.94%   -0.25%     
==========================================
  Files         359      359              
  Lines       30049    30049              
==========================================
- Hits        22894    22820      -74     
- Misses       5578     5642      +64     
- Partials     1577     1587      +10

Impacted Files	Coverage Δ
pkg/transport/timeout_conn.go	80% <0%> (-20%)	⬇️
pkg/adt/interval_tree.go	82.88% <0%> (-10.52%)	⬇️
etcdserver/api/v3rpc/lease.go	81.08% <0%> (-5.41%)	⬇️
etcdserver/corrupt.go	54.42% <0%> (-4.77%)	⬇️
etcdserver/api/v3rpc/watch.go	82.4% <0%> (-3.87%)	⬇️
clientv3/leasing/txn.go	88.09% <0%> (-3.18%)	⬇️
proxy/grpcproxy/watch.go	90.06% <0%> (-3.11%)	⬇️
etcdserver/api/v3election/election.go	64.7% <0%> (-2.95%)	⬇️
clientv3/leasing/cache.go	88.88% <0%> (-2.23%)	⬇️
rafthttp/msgappv2_codec.go	71.3% <0%> (-1.74%)	⬇️
... and 12 more

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[gyuho]

lgtm. Thanks @hexfusion !