test: Avoid Lua closure limit.

Fixes Lua error:
> function at line 543 has more than 60 upvalues

test/functional/helpers.lua

@@ -35,10 +35,6 @@ if nvim_dir == nvim_prog then
   nvim_dir = "."
 end
 
--- Nvim "Unit Under Test" http://en.wikipedia.org/wiki/Device_under_test
-local NvimUUT = {}
-NvimUUT.__index = NvimUUT
-
 local prepend_argv
 
 if os.getenv('VALGRIND') then
@@ -544,11 +540,7 @@ local curbufmeths = create_callindex(curbuf)
 local curwinmeths = create_callindex(curwin)
 local curtabmeths = create_callindex(curtab)
 
-return function(after_each)
-  if after_each then
-    after_each(check_logs)
-  end
-  return {
+local M = {
   prepend_argv = prepend_argv,
   clear = clear,
   connect = connect,
@@ -610,4 +602,10 @@ return function(after_each)
   tmpname = tmpname,
   NIL = mpack.NIL,
 }
+
+return function(after_each)
+  if after_each then
+    after_each(check_logs)
+  end
+  return M
 end