The proc.Stop just sends the SIGTERM to etcd server. The etcd server
needs time to teardown. If we don't wait for it, the etcd server will
hold the port and then next test case, like TestGrpcProxyAutoSync, will
fail to listen on the port 20000.
```bash
file_/__w/etcd/etcd/tests/fixtures/server.crt_--peer-key-file_/__w/etcd/etcd/tests/fixtures/server.key.insecure_--cert-file_/__w/etcd/etcd/tests/fixtures/server2.crt_--key-file_/__w/etcd/etcd/tests/fixtures/server2.key.insecure_--tls-min-version_TLS1.2_--tls-max-version_TLS1.3) (32856): {"level":"info","ts":"2023-10-13T06:12:32.718845Z","caller":"embed/etcd.go:394","msg":"closing etcd server","name":"e1","data-dir":"/tmp/TestEtcdTLSVersion284993522/001","advertise-peer-urls":["https://127.0.0.1:20000"],"advertise-client-urls":["https://0.0.0.0:0"]}
2023-10-13T06:12:32.7399709Z --- PASS: TestEtcdTLSVersion (1.03s)
2023-10-13T06:12:32.7400181Z === RUN TestGrpcProxyAutoSync
2023-10-13T06:12:32.7401203Z logger.go:130: 2023-10-13T06:12:32.719Z INFO starting server... {"name": "TestGrpcProxyAutoSync-test-0"}
2023-10-13T06:12:32.7408306Z logger.go:130: 2023-10-13T06:12:32.719Z INFO spawning process {"args": ["/__w/etcd/etcd/bin/etcd", "--name=TestGrpcProxyAutoSync-test-0", "--listen-client-urls=http://localhost:20000", "--advertise-client-urls=http://localhost:20000", "--listen-peer-urls=http://localhost:20001", "--initial-advertise-peer-urls=http://localhost:20001", "--initial-cluster-token=new", "--data-dir", "/tmp/TestGrpcProxyAutoSync1139610722/001", "--snapshot-count=10000", "--initial-cluster-token=new", "--initial-cluster=TestGrpcProxyAutoSync-test-0=http://localhost:20001", "--initial-cluster-state=new"], "working-dir": "/__w/etcd/etcd/tests/e2e", "name": "TestGrpcProxyAutoSync-test-0", "environment-variables": ["ETCD_VERIFY=all", "EXPECT_DEBUG=true", "PATH=/__t/go/1.21.3/x64/bin:/go/bin:/__t/go/1.21.3/x64/bin:/go/bin:/usr/local/go/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin", "ETCD_UNSUPPORTED_ARCH=amd64"]}
...
2023-10-13T06:12:32.7559855Z /__w/etcd/etcd/bin/etcd (TestGrpcProxyAutoSync-test-0) (32863): {"level":"fatal","ts":"2023-10-13T06:12:32.735191Z","caller":"etcdmain/etcd.go:181","msg":"discovery failed","error":"listen tcp 127.0.0.1:20000: bind: address already in use","stacktrace":"go.etcd.io/etcd/server/v3/etcdmain.startEtcdOrProxyV2\n\tgo.etcd.io/etcd/server/v3/etcdmain/etcd.go:181\ngo.etcd.io/etcd/server/v3/etcdmain.Main\n\tgo.etcd.io/etcd/server/v3/etcdmain/main.go:40\nmain.main\n\tgo.etcd.io/etcd/server/v3/main.go:31\nruntime.main\n\truntime/proc.go:267"}
```
Signed-off-by: Wei Fu <fuweid89@gmail.com>
