[#600] adm/tests: Add missing WaitGroup.Add()

```
panic: sync: negative WaitGroup counter
```

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

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

@@ -75,6 +75,7 @@ func TestGenerateAlphabet(t *testing.T) {
 	var wg sync.WaitGroup
 	for i := uint64(0); i < size; i++ {
 		i := i
+		wg.Add(1)
 		go func() {
 			defer wg.Done()
 			p := filepath.Join(walletDir, innerring.GlagoliticLetter(i).String()+".json")