tests: Use zaptest.NewLogger in tests

[serathius]

cc @ptabor @ahrtr

[ptabor]

Thank you for improving this.

Please fix the failing tests:

stderr: # go.etcd.io/etcd/pkg/v3/flags
Error: stderr: vet: flags/flag_test.go:52:25: undeclared name: zaptest
stderr: # go.etcd.io/etcd/pkg/v3/netutil
Error: stderr: vet: netutil/netutil_test.go:[133](https://github.com/etcd-io/etcd/runs/5785482962?check_suite_focus=true#step:5:133):37: undeclared name: zaptest
stderr: # go.etcd.io/etcd/pkg/v3/osutil
Error: stderr: vet: osutil/osutil_test.go:74:20: undeclared name: zaptest

[codecov-commenter]

Codecov Report

Merging #13867 (d300079) into main (b161093) will decrease coverage by 0.25%.
The diff coverage is n/a.

❗ Current head d300079 differs from pull request most recent head 804fddf. Consider uploading reports for the commit 804fddf to get more accurate results

@@            Coverage Diff             @@
##             main   #13867      +/-   ##
==========================================
- Coverage   72.61%   72.36%   -0.26%     
==========================================
  Files         467      468       +1     
  Lines       38296    38232      -64     
==========================================
- Hits        27809    27665     -144     
- Misses       8714     8774      +60     
- Partials     1773     1793      +20     

Flag	Coverage Δ
all	72.36% <ø> (-0.26%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
raft/raft.go	90.18% <ø> (-0.25%)	⬇️
server/proxy/grpcproxy/register.go	69.76% <0.00%> (-9.31%)	⬇️
server/storage/schema/version.go	73.91% <0.00%> (-8.70%)	⬇️
server/auth/simple_token.go	80.00% <0.00%> (-8.47%)	⬇️
client/v3/namespace/watch.go	87.87% <0.00%> (-6.07%)	⬇️
client/v3/concurrency/mutex.go	61.64% <0.00%> (-5.48%)	⬇️
server/etcdserver/api/v3rpc/watch.go	83.55% <0.00%> (-5.37%)	⬇️
server/etcdserver/api/v3rpc/lease.go	77.21% <0.00%> (-5.07%)	⬇️
raft/rafttest/node.go	95.00% <0.00%> (-5.00%)	⬇️
client/v3/concurrency/session.go	88.63% <0.00%> (-4.55%)	⬇️
... and 22 more

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