Add more MusicXML samples to test suite

[jsbean]

The LilyPond test suite has relatively good coverage, but some real-world examples will be nice. For example, Beethoven string quartets.

This will also raise the stakes w/r/t performance.

[DJBen]

I am working on it to establish a general test suite that test against score parsing.

IMO there are a few examples from the Music XML website worth testing.

Starting from this excerpt from https://www.musicxml.com/tutorial/notation-basics/looks-vs-sounds/, and followed by the example set, we can test this against a dozen real scores.

What do you think of introducing these to score testing? Looking forward to your opinion.

[jsbean]

In fact, I was poking around those myself. I think those are a great place to start.

I think a PR just introducing those tests, along with a slightly-more generalized testing API (i.e., right now we are looking only for LilyPond, but it could be from anywhere) would be amazing.