Added Lua main.c test driver program.

tests/bindings/lua/main.c

@@ -0,0 +1,22 @@
+
+#include <lauxlib.h>
+#include <lua.h>
+#include <lualib.h>
+
+#include "upb/bindings/lua/upb.h"
+
+int main() {
+  int ret = 0;
+  lua_State *L = luaL_newstate();
+  luaL_openlibs(L);
+  lua_pushcfunction(L, luaopen_lupb);
+
+  if (luaL_dostring(L, "package.preload['lupb'] = ...[1]") ||
+      luaL_dofile(L, "tests/bindings/lua/test_upb.lua")) {
+    fprintf(stderr, "error testing Lua: %s\n", lua_tostring(L, -1));
+    ret = 1;
+  }
+
+  lua_close(L);
+  return ret;
+}