build: migrate test framework from plenary to busted

Problem: plenary.nvim is deprecated. The test suite depends on
plenary's async test runner and coroutine-based utilities, tying the
project to an unmaintained dependency. CI also tests against Neovim
0.8-0.11, which are no longer relevant.

Solution: replace plenary with busted + nlua (nvim -l). Convert all
async test patterns (a.wrap, a.util.sleep, a.util.scheduler) to
synchronous equivalents using vim.wait. Rename tests/ to spec/ to
follow busted convention. Replace the CI test matrix with
nvim-busted-action targeting stable/nightly only. Add .busted config,
luarocks test_dependencies, and update the nix devshell.

tests/test_util.lua

@@ -1,140 +0,0 @@
-require('plenary.async').tests.add_to_env()
-local cache = require('oil.cache')
-local test_adapter = require('oil.adapters.test')
-local util = require('oil.util')
-local M = {}
-
-M.reset_editor = function()
-  require('oil').setup({
-    columms = {},
-    adapters = {
-      ['oil-test://'] = 'test',
-    },
-    prompt_save_on_select_new_entry = false,
-  })
-  vim.cmd.tabonly({ mods = { silent = true } })
-  for i, winid in ipairs(vim.api.nvim_tabpage_list_wins(0)) do
-    if i > 1 then
-      vim.api.nvim_win_close(winid, true)
-    end
-  end
-  vim.api.nvim_win_set_buf(0, vim.api.nvim_create_buf(false, true))
-  for _, bufnr in ipairs(vim.api.nvim_list_bufs()) do
-    vim.api.nvim_buf_delete(bufnr, { force = true })
-  end
-  cache.clear_everything()
-  test_adapter.test_clear()
-end
-
-local function throwiferr(err, ...)
-  if err then
-    error(err)
-  else
-    return ...
-  end
-end
-
-M.oil_open = function(...)
-  a.wrap(require('oil').open, 3)(...)
-end
-
-M.await = function(fn, nargs, ...)
-  return throwiferr(a.wrap(fn, nargs)(...))
-end
-
-M.wait_for_autocmd = a.wrap(function(autocmd, cb)
-  local opts = {
-    pattern = '*',
-    nested = true,
-    once = true,
-  }
-  if type(autocmd) == 'table' then
-    opts = vim.tbl_extend('force', opts, autocmd)
-    autocmd = autocmd[1]
-    opts[1] = nil
-  end
-  opts.callback = vim.schedule_wrap(cb)
-
-  vim.api.nvim_create_autocmd(autocmd, opts)
-end, 2)
-
-M.wait_oil_ready = a.wrap(function(cb)
-  util.run_after_load(0, vim.schedule_wrap(cb))
-end, 1)
-
----@param actions string[]
----@param timestep integer
-M.feedkeys = function(actions, timestep)
-  timestep = timestep or 10
-  a.util.sleep(timestep)
-  for _, action in ipairs(actions) do
-    a.util.sleep(timestep)
-    local escaped = vim.api.nvim_replace_termcodes(action, true, false, true)
-    vim.api.nvim_feedkeys(escaped, 'm', true)
-  end
-  a.util.sleep(timestep)
-  -- process pending keys until the queue is empty.
-  -- Note that this will exit insert mode.
-  vim.api.nvim_feedkeys('', 'x', true)
-  a.util.sleep(timestep)
-end
-
-M.actions = {
-  ---Open oil and wait for it to finish rendering
-  ---@param args string[]
-  open = function(args)
-    vim.schedule(function()
-      vim.cmd.Oil({ args = args })
-      -- If this buffer was already open, manually dispatch the autocmd to finish the wait
-      if vim.b.oil_ready then
-        vim.api.nvim_exec_autocmds('User', {
-          pattern = 'OilEnter',
-          modeline = false,
-          data = { buf = vim.api.nvim_get_current_buf() },
-        })
-      end
-    end)
-    M.wait_for_autocmd({ 'User', pattern = 'OilEnter' })
-  end,
-
-  ---Save all changes and wait for operation to complete
-  save = function()
-    vim.schedule_wrap(require('oil').save)({ confirm = false })
-    M.wait_for_autocmd({ 'User', pattern = 'OilMutationComplete' })
-  end,
-
-  ---@param bufnr? integer
-  reload = function(bufnr)
-    M.await(require('oil.view').render_buffer_async, 3, bufnr or 0)
-  end,
-
-  ---Move cursor to a file or directory in an oil buffer
-  ---@param filename string
-  focus = function(filename)
-    local lines = vim.api.nvim_buf_get_lines(0, 0, -1, true)
-    local search = ' ' .. filename .. '$'
-    for i, line in ipairs(lines) do
-      if line:match(search) then
-        vim.api.nvim_win_set_cursor(0, { i, 0 })
-        return
-      end
-    end
-    error('Could not find file ' .. filename)
-  end,
-}
-
----Get the raw list of filenames from an unmodified oil buffer
----@param bufnr? integer
----@return string[]
-M.parse_entries = function(bufnr)
-  bufnr = bufnr or 0
-  if vim.bo[bufnr].modified then
-    error("parse_entries doesn't work on a modified oil buffer")
-  end
-  local lines = vim.api.nvim_buf_get_lines(bufnr, 0, -1, true)
-  return vim.tbl_map(function(line)
-    return line:match('^/%d+ +(.+)$')
-  end, lines)
-end
-
-return M