[test] Update reference data for IT portamento envelope reset

For some reason reference data was not correct if compared to IT2.14v5.
Maybe it was validated by comparing to Schism Tracker?

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Fix IT portamento and fadeout on new note

These had to be fixed after revising IT test cases against IT2.14v5, It
seems that previous test cases were checked against Schism Tracker.

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

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Fix IT portamento after keyoff

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Fix IT compatible Gxx portamento after keyoff

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
[test] Update portamento tests to check mixer data

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>