Tests for saving ID3v2.3

[JuniorJPDJ]

I think we should have some tests for this as It's supported but not checked if works. I'm pretty sure big part of multi-valued tags would fail and this should be only expected failure.

[sampsyo]

That seems like a good idea. For exceptions, this seems straightforward. For other (non-failure) cases, would we just check "round-trip" preservation of data?

[JuniorJPDJ]

I was thinking about using same test set as for other formats, just change saving to id3v2.3 for new "format". Would also need to set some failures as expected but it doesn't seem to hard to do.