e2e: deflake TestCtlV3AuthGracefulDisable

We should call Wait for grpc-proxy process stop before start. Otherwise,
the tcp port won't be released.

Fixes: etcd-io#14926

Signed-off-by: Wei Fu <fuweid89@gmail.com>

tests/framework/e2e/cluster_proxy.go

@@ -159,10 +159,21 @@ func (pp *proxyProc) Stop() error {
 	if pp.proc == nil {
 		return nil
 	}
-	if err := pp.proc.Stop(); err != nil && !strings.Contains(err.Error(), "exit status 1") {
-		// v2proxy exits with status 1 on auto tls; not sure why
+	err := pp.proc.Stop()
+	if err != nil {
 		return err
 	}
+
+	err = pp.proc.Close()
+	if err != nil {
+		// proxy received SIGTERM signal
+		if !(strings.Contains(err.Error(), "unexpected exit code") ||
+			// v2proxy exits with status 1 on auto tls; not sure why
+			strings.Contains(err.Error(), "exit status 1")) {
+
+			return err
+		}
+	}
 	pp.proc = nil
 	<-pp.donec
 	pp.donec = make(chan struct{})