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.

spec/regression_spec.lua

@@ -0,0 +1,137 @@
+local TmpDir = require('spec.tmpdir')
+local actions = require('oil.actions')
+local oil = require('oil')
+local test_util = require('spec.test_util')
+local view = require('oil.view')
+
+describe('regression tests', function()
+  local tmpdir
+  before_each(function()
+    tmpdir = TmpDir.new()
+  end)
+  after_each(function()
+    if tmpdir then
+      tmpdir:dispose()
+      tmpdir = nil
+    end
+    test_util.reset_editor()
+  end)
+
+  it('can edit dirs that will be renamed to an existing buffer', function()
+    vim.cmd.edit({ args = { 'README.md' } })
+    vim.cmd.vsplit()
+    vim.cmd.edit({ args = { '%:p:h' } })
+    assert.equals('oil', vim.bo.filetype)
+    vim.cmd.wincmd({ args = { 'p' } })
+    assert.equals('markdown', vim.bo.filetype)
+    vim.cmd.edit({ args = { '%:p:h' } })
+    test_util.wait_for_autocmd({ 'User', pattern = 'OilEnter' })
+    assert.equals('oil', vim.bo.filetype)
+  end)
+
+  it('places the cursor on correct entry when opening on file', function()
+    vim.cmd.edit({ args = { '.' } })
+    test_util.wait_for_autocmd({ 'User', pattern = 'OilEnter' })
+    local entry = oil.get_cursor_entry()
+    assert.not_nil(entry)
+    assert.not_equals('README.md', entry and entry.name)
+    vim.cmd.edit({ args = { 'README.md' } })
+    view.delete_hidden_buffers()
+    oil.open()
+    test_util.wait_for_autocmd({ 'User', pattern = 'OilEnter' })
+    entry = oil.get_cursor_entry()
+    assert.equals('README.md', entry and entry.name)
+  end)
+
+  it("doesn't close floating windows oil didn't open itself", function()
+    local winid = vim.api.nvim_open_win(vim.fn.bufadd('README.md'), true, {
+      relative = 'editor',
+      row = 1,
+      col = 1,
+      width = 100,
+      height = 100,
+    })
+    oil.open()
+    vim.wait(10)
+    oil.close()
+    vim.wait(10)
+    assert.equals(winid, vim.api.nvim_get_current_win())
+  end)
+
+  it("doesn't close splits on oil.close", function()
+    vim.cmd.edit({ args = { 'README.md' } })
+    vim.cmd.vsplit()
+    local winid = vim.api.nvim_get_current_win()
+    local bufnr = vim.api.nvim_get_current_buf()
+    oil.open()
+    vim.wait(10)
+    oil.close()
+    vim.wait(10)
+    assert.equals(2, #vim.api.nvim_tabpage_list_wins(0))
+    assert.equals(winid, vim.api.nvim_get_current_win())
+    assert.equals(bufnr, vim.api.nvim_get_current_buf())
+  end)
+
+  it('Returns to empty buffer on close', function()
+    oil.open()
+    test_util.wait_for_autocmd({ 'User', pattern = 'OilEnter' })
+    oil.close()
+    assert.not_equals('oil', vim.bo.filetype)
+    assert.equals('', vim.api.nvim_buf_get_name(0))
+  end)
+
+  it('All buffers set nomodified after save', function()
+    tmpdir:create({ 'a.txt' })
+    vim.cmd.edit({ args = { 'oil://' .. vim.fn.fnamemodify(tmpdir.path, ':p') } })
+    local first_dir = vim.api.nvim_get_current_buf()
+    test_util.wait_for_autocmd({ 'User', pattern = 'OilEnter' })
+    test_util.feedkeys({ 'dd', 'itest/<esc>', '<CR>' }, 10)
+    vim.wait(1000, function()
+      return vim.bo.modifiable
+    end, 10)
+    test_util.feedkeys({ 'p' }, 10)
+    oil.save({ confirm = false })
+    vim.wait(1000, function()
+      return vim.bo.modifiable
+    end, 10)
+    tmpdir:assert_fs({
+      ['test/a.txt'] = 'a.txt',
+    })
+    assert.falsy(vim.bo[first_dir].modified)
+  end)
+
+  it("refreshing buffer doesn't lose track of it", function()
+    vim.cmd.edit({ args = { '.' } })
+    test_util.wait_for_autocmd({ 'User', pattern = 'OilEnter' })
+    local bufnr = vim.api.nvim_get_current_buf()
+    vim.cmd.edit({ bang = true })
+    test_util.wait_for_autocmd({ 'User', pattern = 'OilEnter' })
+    assert.are.same({ bufnr }, require('oil.view').get_all_buffers())
+  end)
+
+  it('can copy a file multiple times', function()
+    test_util.actions.open({ tmpdir.path })
+    vim.api.nvim_feedkeys('ifoo.txt', 'x', true)
+    test_util.actions.save()
+    vim.api.nvim_feedkeys('yyp$ciWbar.txt', 'x', true)
+    vim.api.nvim_feedkeys('yyp$ciWbaz.txt', 'x', true)
+    test_util.actions.save()
+    assert.are.same({ 'bar.txt', 'baz.txt', 'foo.txt' }, test_util.parse_entries(0))
+    tmpdir:assert_fs({
+      ['foo.txt'] = '',
+      ['bar.txt'] = '',
+      ['baz.txt'] = '',
+    })
+  end)
+
+  it('can open files from floating window', function()
+    tmpdir:create({ 'a.txt' })
+    oil.open_float(tmpdir.path)
+    test_util.wait_for_autocmd({ 'User', pattern = 'OilEnter' })
+    actions.select.callback()
+    vim.wait(1000, function()
+      return vim.fn.expand('%:t') == 'a.txt'
+    end, 10)
+    assert.equals('a.txt', vim.fn.expand('%:t'))
+  end)
+end)