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/manual_progress.lua

@@ -0,0 +1,28 @@
+-- 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, {})