Add semantic tests for the MusicXML export

[wang1zhen]

Add semantic tests for the MusicXML export

This is the first of four PRs that improve the LilyPond to MusicXML export (ly.musicxml). It only adds tests; the following PRs build on it:

1. this PR: tests
2. MusicXML export: don't hide errors, write valid MusicXML 4.0 #202: errors are no longer hidden, the output is valid MusicXML 4.0
3. MusicXML export: measures, divisions and grace notes #200: measures, durations and grace notes
4. MusicXML export: transpose, repeats, chord mode, lyrics and partCombine #201: \transpose, repeats, chord mode, lyrics and \partCombine

What it adds

The existing tests compare whole files, so a change anywhere in the output makes them fail, and they cover few features. The new tests look only at the part of the output they are about: pitches, voices, measure lengths, divisions, barlines and so on. Helpers for this are in tests/musicxml_helpers.py.

• One test module per area of the export: dispatch of ly.music nodes, \transpose, repeats, chord mode, \afterGrace, \partCombine, lyrics, measures, divisions, and the MusicXML version.
• A corpus of realistic snippets (tests/musicxml_corpus.py): every snippet must convert without warnings to MusicXML that is valid against the schema.
• The MusicXML 4.0 schema, localized like the existing 3.0 schema (tests/musicxml-4.0/).

Tests for behaviour that does not work yet are marked as strict xfail. Each of the following PRs removes the markers of the tests it makes pass, so its effect is visible in the test results.

Writing these tests found several bugs in the current export, among them:

• a measure ends in the middle of a triplet
• the overflow of a note that crosses a barline is dropped
• \transpose is ignored
• the forward repeat is put on the right barline
• the stanzas of the lyrics are all numbered 1
• \scaleDurations crashes
• the DOCTYPE says MusicXML 2.0

All of these are fixed in the following PRs.

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