etcdserver: add failpoints walBeforeSync and walAfterSync

[ahrtr]

It can be used to reproduce #15247

Signed-off-by: Benjamin Wang wachao@vmware.com

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

[codecov-commenter]

Codecov Report

Merging #15252 (fcb4172) into main (8f54d38) will decrease coverage by 0.23%.
The diff coverage is 100.00%.

📣 This organization is not using Codecov’s GitHub App Integration. We recommend you install it so Codecov can continue to function properly for your repositories. Learn more

@@            Coverage Diff             @@
##             main   #15252      +/-   ##
==========================================
- Coverage   74.92%   74.70%   -0.23%     
==========================================
  Files         416      416              
  Lines       34351    34352       +1     
==========================================
- Hits        25736    25661      -75     
- Misses       6988     7048      +60     
- Partials     1627     1643      +16     

Flag	Coverage Δ
all	74.70% <100.00%> (-0.23%)	⬇️

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

Impacted Files	Coverage Δ
server/storage/wal/wal.go	57.45% <100.00%> (+0.07%)	⬆️
server/proxy/grpcproxy/register.go	69.76% <0.00%> (-20.94%)	⬇️
client/v3/leasing/util.go	91.66% <0.00%> (-6.67%)	⬇️
client/v3/namespace/watch.go	87.87% <0.00%> (-6.07%)	⬇️
client/pkg/v3/testutil/leak.go	62.83% <0.00%> (-4.43%)	⬇️
server/etcdserver/api/v3rpc/watch.go	83.49% <0.00%> (-4.13%)	⬇️
client/v3/leasing/cache.go	87.77% <0.00%> (-3.89%)	⬇️
server/etcdserver/txn/util.go	96.42% <0.00%> (-3.58%)	⬇️
server/etcdserver/api/rafthttp/msgappv2_codec.go	71.30% <0.00%> (-3.48%)	⬇️
server/etcdserver/api/v3rpc/interceptor.go	74.47% <0.00%> (-3.13%)	⬇️
... and 14 more

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[fuweid]

LGTM