Specify Lua version to install in testing docs

[clorl]

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Spent a whole lot of time banging my head on why testing errored. This is because you need to install Lua 5.1 system-wide even if in the end you use nlua to make neovim your interpreter. This is because when luarocks installs some rocks, it may compile some of them to shared objects against the lua version you have installed globally on your system. This means that if you have luarocks + lua > 5.1 on your system, buster can't even run.

The error I got was on pl/path.lua where it couldn't require luafilesystem, even though the .so file was present. lfs.so failed because it was using undefined symbols that aren't available in Lua 5.1.