integration: test large request response back from server

[gyuho]

Address #9043.
Won't fix it, but we need test coverage on response back
from server as well.

Will fix the issue in the following PR.

/cc @jpbetz @xiang90

[codecov-io]

Codecov Report

Merging #9046 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9046      +/-   ##
==========================================
+ Coverage   76.08%   76.09%   +0.01%     
==========================================
  Files         359      359              
  Lines       29841    29841              
==========================================
+ Hits        22704    22707       +3     
+ Misses       5559     5556       -3     
  Partials     1578     1578

Impacted Files	Coverage Δ
clientv3/leasing/util.go	91.66% <0%> (-6.67%)	⬇️
pkg/testutil/recorder.go	77.77% <0%> (-3.71%)	⬇️
pkg/adt/interval_tree.go	82.88% <0%> (-3.61%)	⬇️
proxy/grpcproxy/watch.go	91.92% <0%> (-3.11%)	⬇️
clientv3/health_balancer.go	85.97% <0%> (-1.53%)	⬇️
mvcc/watchable_store.go	90.03% <0%> (ø)	⬆️
lease/leasehttp/http.go	63.97% <0%> (ø)	⬆️
etcdserver/api/v3rpc/lease.go	83.82% <0%> (ø)	⬆️
clientv3/namespace/watch.go	93.93% <0%> (ø)	⬆️
clientv3/watch.go	95.04% <0%> (ø)	⬆️
... and 9 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 74a600a...abfc09b. Read the comment docs.

[xiang90]

lgtm

[jpbetz]

lgtm as well