Fixed LuaJIT when not compiled with COMPAT mode (#6605)

LuaJIT as installed by apt doesn't include the COMPAT mode for using
Lua5.2 features. So this change just makes the flatbuffer code use a
common implementation.

tests/LuaTest.sh

@@ -19,7 +19,13 @@ test_dir="$(pwd)"
 
 ${test_dir}/../flatc --lua -I include_test monster_test.fbs
 
-echo "Run with LuaJIT:"
-luajit luatest.lua
-echo "Run with Lua 5.3:"
-lua5.3 luatest.lua
+declare -a versions=(luajit lua5.3)
+
+for i in "${versions[@]}"
+do
+    if command -v $i &> /dev/null
+    then
+        echo "[$i]"
+        $i luatest.lua
+    fi
+done