[#144] registry: Fix string generator func in obj info test

`rune` is alias for `int32`. `randString()` used `rand.Int()`
which returns system-specific non-negative integer number. For
64-bit systems it will be int64. An attempt to cast `int64` to
`int32` (`rune`) leads the latter to be a negative number in
case of overflow. This caused the resulting string containing
unexpected symbols.

Signed-off-by: Ekaterina Lebedeva <ekaterina.lebedeva@yadro.com>

internal/registry/obj_info_test.go

@@ -101,7 +101,7 @@ func randomObjectInfo() ObjectInfo {
 func randString(n int) string {
 	var sb strings.Builder
 	for i := 0; i < n; i++ {
-		sb.WriteRune('a' + rune(rand.Int())%('z'-'a'+1))
+		sb.WriteRune('a' + rune(rand.Int31())%('z'-'a'+1))
 	}
 	return sb.String()
 }