[test] Add OpenMPT FineVolColSlide.it test case

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Change sample in IT if sample changes in tone portamento

Impulse tracker uses the mapped sample to determine if sample should be
changed during a tone portamento. I think we should refactor the entire
event reader based on this, but let's fix the behavior first based on
OpenMPT's PortaInsNum.it test case.

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