Add missing test script.

tests/test_join_gc.lua

@@ -0,0 +1,16 @@
+local llthreads = require "llthreads"
+
+local CODE=[[
+local function gc(fn)
+  local p = assert(newproxy())
+  assert(debug.setmetatable(p, { __gc = fn }))
+  return p
+end 
+
+p = gc(function() print("hello from gc") end)
+]]
+
+local thread = llthreads.new(CODE)
+thread:start()
+thread:join()
+