Add 'Buffer' global for GCS unit tests

services/filestore/test/unit/js/GcsPersistorTests.js

@@ -138,7 +138,7 @@ describe('GcsPersistorTests', function() {
         'metrics-sharelatex': Metrics,
         crypto
       },
-      globals: { console }
+      globals: { console, Buffer }
     })
   })