added test for vibrato... and fixed vibrato

naturally when i actually tested it, it turned out to be subtly broken

also reorg'd the tests a bit, sorry @hillerstorm.

test/testdata.js

@@ -22,6 +22,7 @@ exports.resetXMData = function() {
     mute: 0,
     volE: 0, panE: 0,
     retrig: 0,
+    vibratopos: 0,
     vibratodepth: 1,
     vibratospeed: 1,
   });