clientv3/integration: log test failures from slow balancer as TODO

[gyuho]

Balancer tests have been flaky since last balancer rewrite.
Should be from slow balancer failover. Once we re-rewrite balancer
with roundrobin, there should be no more flaky tests as below:

TestBalancerUnderServerStopInflightLinearizableGetOnRestart (27.62s)
server_shutdown_test.go:343: context deadline exceeded

TestBalancerUnderServerShutdownDelete (3.22s)
util.go:32: context deadline exceeded

/cc @jpbetz

[codecov-io]

Codecov Report

Merging #9555 into master will decrease coverage by 0.13%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #9555      +/-   ##
==========================================
- Coverage    72.5%   72.36%   -0.14%     
==========================================
  Files         368      368              
  Lines       31339    31339              
==========================================
- Hits        22721    22678      -43     
- Misses       6982     7029      +47     
+ Partials     1636     1632       -4

Impacted Files	Coverage Δ
clientv3/integration/util.go	66.66% <100%> (ø)	⬆️
pkg/transport/timeout_conn.go	80% <0%> (-20%)	⬇️
client/client.go	70.26% <0%> (-14.38%)	⬇️
pkg/adt/interval_tree.go	83.48% <0%> (-6.91%)	⬇️
mvcc/watcher.go	96.29% <0%> (-3.71%)	⬇️
clientv3/balancer/grpc1.7-health.go	85.17% <0%> (-1.75%)	⬇️
proxy/grpcproxy/lease.go	79.18% <0%> (-1.36%)	⬇️
proxy/grpcproxy/watch.go	86.33% <0%> (-1.25%)	⬇️
pkg/proxy/server.go	58.81% <0%> (ø)	⬆️
etcdserver/server.go	79.8% <0%> (+0.22%)	⬆️
... and 10 more

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