fix(vim.fs): dirname() returns "." on mingw/msys2 #30480
Problem:
`vim.fs.dirname([[C:\User\XXX\AppData\Local]])` returns "." on
mingw/msys2.
Solution:
- Check for "mingw" when deciding `iswin`.
- Use `has("win32")` where possible, it works in "fast" contexts since
b02eeb6a72.
Specifically, functions that are run in the context of the test runner
are put in module `test/testutil.lua` while the functions that are run
in the context of the test session are put in
`test/functional/testnvim.lua`.
Closes https://github.com/neovim/neovim/issues/27004.
