[test] Update keyoff after portamento test

Improve portamento tests to cover note without instrument and instrument
without note cases.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Change IT event read to cover special cases

IT event read changed to cover instrument without note and note without
instrument cases.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Refactor event read code

Remove redundant tests and unnecessary temporary variables.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
[test] Update IT portamento after keyoff tests

Only test the first part (note and instrument). IT and schism don't agree
on note without instrument and instrument without note parts, and our result
doesn't match any of them.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Revert "Code cleanup: remove unnecessary flag from IT event read"

This reverts commit ddac8f630dc3f9b4721f0c576d7fafb72bb4f35c.

Actually this was necessary.
Revert "Change IT event read to cover special cases"

This reverts commit 5cc1806a0d2284e774608c6799df133d6bd5ce2c.