private static Optional<Integer> maybeNumber(String str) {
        try {
            return Optional.of(Integer.parseInt(str));
        } catch (NumberFormatException e) {
            return Optional.empty();
        }
    }
Same as {@code com.google.common.jimfs.UnixAttributeProvider} except that getUniqueId() will return user
if user is a numerical string.
testutil/src/main/java/com/yahoo/vespa/test/file/UnixUidGidAttributeProvider.java