test/mbyte_spec: skip broken test on QuickBuild

Forgot `return` in eec529cf9e.

test/unit/mbyte_spec.lua

@@ -10,6 +10,7 @@ local charset = helpers.cimport('./src/nvim/charset.h')
 describe('mbyte', function()
   if helpers.isCI('quickbuild') then
     pending("crashes on quickbuild", function() end)
+    return
   end
 
   -- Array for composing characters