[test] Fix test cases for volume up+down

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Fix volume slide up+down processing

Change the default behavior to use "priority up" when both volume slide
up and down are specified. S3M and IT have a quirk to use "priority down".

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