[#1138] adm: Parallelize generate-alphabet command

This is the longest test in our suite, try to help CI a bit.

Before:
```
$ go test -run=TestGenerateAlphabet -count=10 .
ok      git.frostfs.info/TrueCloudLab/frostfs-node/cmd/frostfs-adm/internal/modules/morph/generate      45.400s
```

After:
```
$ go test -run=TestGenerateAlphabet -count=10 .
ok      git.frostfs.info/TrueCloudLab/frostfs-node/cmd/frostfs-adm/internal/modules/morph/generate      33.267s
```

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

cmd/frostfs-adm/internal/modules/morph/generate/generate.go

@@ -65,34 +65,42 @@ func initializeWallets(v *viper.Viper, walletDir string, size int) ([]string, er
 	pubs := make(keys.PublicKeys, size)
 	passwords := make([]string, size)
 
+	var errG errgroup.Group
+
 	for i := range wallets {
 		password, err := config.GetPassword(v, innerring.GlagoliticLetter(i).String())
 		if err != nil {
 			return nil, fmt.Errorf("can't fetch password: %w", err)
 		}
 
+		i := i
+		errG.Go(func() error {
 			p := filepath.Join(walletDir, innerring.GlagoliticLetter(i).String()+".json")
 			f, err := os.OpenFile(p, os.O_CREATE, 0o644)
 			if err != nil {
-			return nil, fmt.Errorf("can't create wallet file: %w", err)
+				return fmt.Errorf("can't create wallet file: %w", err)
 			}
 			if err := f.Close(); err != nil {
-			return nil, fmt.Errorf("can't close wallet file: %w", err)
+				return fmt.Errorf("can't close wallet file: %w", err)
 			}
 			w, err := wallet.NewWallet(p)
 			if err != nil {
-			return nil, fmt.Errorf("can't create wallet: %w", err)
+				return fmt.Errorf("can't create wallet: %w", err)
 			}
 			if err := w.CreateAccount(constants.SingleAccountName, password); err != nil {
-			return nil, fmt.Errorf("can't create account: %w", err)
+				return fmt.Errorf("can't create account: %w", err)
 			}
 
 			passwords[i] = password
 			wallets[i] = w
 			pubs[i] = w.Accounts[0].PrivateKey().PublicKey()
+			return nil
+		})
 	}
 
-	var errG errgroup.Group
+	if err := errG.Wait(); err != nil {
+		return nil, err
+	}
 
 	// Create committee account with N/2+1 multi-signature.
 	majCount := smartcontract.GetMajorityHonestNodeCount(size)