etcd-tester: fix "writeTxn" key selection

[gyuho]

Found when debugging #9130.

[codecov-io]

Codecov Report

Merging #9133 into master will decrease coverage by 0.25%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #9133      +/-   ##
=========================================
- Coverage   76.25%     76%   -0.26%     
=========================================
  Files         359     359              
  Lines       29983   29983              
=========================================
- Hits        22864   22789      -75     
- Misses       5541    5605      +64     
- Partials     1578    1589      +11

Impacted Files	Coverage Δ
pkg/transport/timeout_conn.go	80% <0%> (-20%)	⬇️
proxy/grpcproxy/register.go	69.44% <0%> (-13.89%)	⬇️
pkg/fileutil/purge.go	73.68% <0%> (-7.9%)	⬇️
clientv3/namespace/watch.go	93.93% <0%> (-6.07%)	⬇️
etcdctl/ctlv3/command/lease_command.go	65.34% <0%> (-5.95%)	⬇️
etcdserver/api/v3rpc/lease.go	81.08% <0%> (-5.41%)	⬇️
clientv3/concurrency/session.go	88.63% <0%> (-4.55%)	⬇️
proxy/grpcproxy/watch.go	90.06% <0%> (-4.35%)	⬇️
pkg/netutil/netutil.go	63.21% <0%> (-3.45%)	⬇️
etcdserver/api/v3rpc/watch.go	85.4% <0%> (-2.58%)	⬇️
... and 14 more

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