proxy: fix flaky TestAuditLogging (#2575)

The TestAuditLogging could flake because it checks audit log after
receiving response message from the connection which may happen
before write to audit log by io.MultiWriter has finished.

It could be reproduced via:
```
go test ./proxy -run=TestAuditLogging -count=10000 -failfast
```

This change writes to auditLog before writing to the connection which
also would show failed message in the audit log in case write to
connection fails.

Fixes #2572

Signed-off-by: Alexander Yastrebov <[email protected]>

proxy/upgrade.go

@@ -161,7 +161,7 @@ func (p *upgradeProxy) serveHTTP(w http.ResponseWriter, req *http.Request) {
 	done := make(chan struct{}, 2)
 
 	if p.useAuditLog {
-		copyAsync("backend->request+audit", backendConn, io.MultiWriter(requestHijackedConn, p.auditLogOut), done)
+		copyAsync("backend->request+audit", backendConn, io.MultiWriter(p.auditLogOut, requestHijackedConn), done)
 	} else {
 		copyAsync("backend->request", backendConn, requestHijackedConn, done)
 	}

proxy/upgrade_test.go

@@ -344,10 +344,9 @@ func TestAuditLogging(t *testing.T) {
 		return func(t *testing.T) {
 			wss := httptest.NewServer(websocket.Handler(func(ws *websocket.Conn) {
 				if _, err := io.Copy(ws, ws); err != nil {
-					t.Fatal(err)
+					t.Error(err)
 				}
 			}))
-
 			defer wss.Close()
 
 			// only used as poor man's sync, the audit log in question goes stdout and stderr,