test: new test for LuaJIT fold machinery

https://github.com/LuaJIT/LuaJIT/issues/505

test/luajit-tap/fold_bug_LuaJIT_505.test.lua

+#!/usr/bin/env tarantool
+
+tap = require('tap')
+
+test = tap.test("505")
+test:plan(1)
+
+-- Test file to demonstrate Lua fold machinery icorrect behavior, details:
+--     https://github.com/LuaJIT/LuaJIT/issues/505
+
+jit.opt.start("hotloop=1")
+for _ = 1, 20 do
+    local value = "abc"
+    local pos_c = string.find(value, "c", 1, true)
+    local value2 = string.sub(value, 1, pos_c - 1)
+    local pos_b = string.find(value2, "b", 2, true)
+    assert(pos_b == 2, "FAIL: position of 'b' is " .. pos_b)
+end
+
+test:ok("PASS")