Add GitHub Action workflow to run lua tests/test_py.lua

[cclauss]

Add a GitHub Action to run lua tests/test_py.lua so we can focus on fixes for:

• lua5.2 only: python.so: undefined symbol: luaL_loadbuffer #98
• lua5.4 only: python.so: undefined symbol: lua_insert #99
• lua5.5 only: python.so: undefined symbol: luaL_openlibs #100

- if: matrix.lua-version == '5.1' || matrix.lua-version == '5.3'
  run: lua${{ matrix.lua-version }} tests/test_py.lua

All 10 GHA runs pass on both ubuntu-26.04 and ubuntu-26.04-arm.

I believe that this is the best GitHub Actions PR to review and merge. It runs a test that passes and can provide feedback for three issues that can then be fixed and closed. Once this is merged, we will have Lua builds with continuous integration basic guardrails in place, and any other GHA PRs can be rebased.

[cclauss]

@bastibe @gward @leso-kn @shmuz @skorokithakis Your reviews, please?

• https://github.com/bastibe/lunatic-python/pulls