add test for duet headings

[finnboeger]

Blocked on #8

[man0lis]

This is not a test for duet header tags, this is a test for handling unknown tags.
If you want support for duet headers, please add them to the to the header struct and parse them.
The duet headers are already defined in a comment in src/structs.rs:99.
I'm not sure if songs exist, that support more than 2 singers. Maybe one could use a Vec of players if the number of possible singers is not fixed.

[finnboeger]

I will have a look at adding support for some of the currently unknown tags. A, possibly incomplete, list of them here:

• Duet Header Tags: P1, P2, DUETSINGERP1, DUETSINGERP2
• (CREATOR, AUTHOR, AUTHORS)
• MEDLEYSTARTBEAT, MEDLEYENDBEAT
• ENCODE, ENCODING
• PREVIEWSTART
• COMMENT
• UPDATE
• START
• END
• ALBUM

I have never encountered a song with more than two singers, so I don't think that would be necessary.

[finnboeger]

Frequently used (and relevant) tags are introduced in #12 together with relevant tests. This is therefore unneeded.