This is a better way to prevent parallel tests from interfering with
each other, as there are many ways files can be created and deleted in
tests, so enforcing different file names is hard.
Using $TMPDIR can also work in most cases, but 'backipskip' etc. have
special defaults for $TMPDIR.
Symlink runtime/, src/, test/ and README.md to Xtest_xdg dir to make
tests more convenient (and symlinking test/ is required for busted).
Also, use README.md instead of test/README.md in the Ex mode inccommand
test, as test/README.md no longer contains 'N' char.
Problem:
Current `'includeexpr'` in runtime/ftplugin/lua.vim doesn't work with Nvim Lua.
Solution:
Provide an improved 'includeexpr' for Lua in "ftplugin/lua.lua".
Closes: https://github.com/neovim/neovim/issues/32490