group= and off_by= tests

[pljones]

With a quick skim through the names of the test files, I didn't spot and group / off_by tests.

Here's the René SFZ v1 version:

Any layer to cause muting is labelled with group (0 to 4Gb). Triggering such a layer causes any layer with a different trigger event and a matching, non-zero off_by value to be muted or released.

GIVEN one `<region>`s with the same `off_by=` value
AND a second `<region>` with a `group=` value that matches the first region's `off_by=` value
AND the second `<region>` region is triggered by a different note number
AND the first region has been triggered and is still playing
WHEN the second region is triggered
THEN the first region should stop, using the `off_mode=` value

GIVEN one `<region>`s with the same `off_by=` value
AND a second `<region>` with a `group=` value that matches the first region's `off_by=` value
AND the second `<region>` region is triggered by the same note number
AND the first region has been triggered and is still playing
WHEN the second region is triggered
THEN the first region should not be affected

Here's the "OK, you win" v2 version:

GIVEN one `<region>`s with the same `off_by=` value
AND a second `<region>` with a `group=` value that matches the first region's `off_by=` value
AND the second `<region>` region is triggered by a different note number
AND the first region has been triggered and is still playing
WHEN the second region is triggered
THEN the first region should stop, using the `off_mode=` value

GIVEN one `<region>`s with the same `off_by=` value
AND a second `<region>` with a `group=` value that matches the first region's `off_by=` value
AND the second `<region>` region is triggered by the same note number
AND the first region has been triggered and is still playing
WHEN the second region is triggered
THEN the first region should stop, using the `off_mode=` value