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/manual_progress.lua

@@ -1,28 +0,0 @@
--- Manual test for minimizing/restoring progress window
-local Progress = require('oil.mutator.progress')
-
-local progress = Progress.new()
-
-progress:show({
-  cancel = function()
-    progress:close()
-  end,
-})
-
-for i = 1, 10, 1 do
-  vim.defer_fn(function()
-    progress:set_action({
-      type = 'create',
-      url = string.format('oil:///tmp/test_%d.txt', i),
-      entry_type = 'file',
-    }, i, 10)
-  end, (i - 1) * 1000)
-end
-
-vim.defer_fn(function()
-  progress:close()
-end, 10000)
-
-vim.keymap.set('n', 'R', function()
-  progress:restore()
-end, {})