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/url_spec.lua

@@ -0,0 +1,25 @@
+local oil = require('oil')
+local util = require('oil.util')
+describe('url', function()
+  it('get_url_for_path', function()
+    local cases = {
+      { '', 'oil://' .. util.addslash(vim.fn.getcwd()) },
+      { 'term://~/oil.nvim//52953:/bin/sh', 'oil://' .. vim.loop.os_homedir() .. '/oil.nvim/' },
+      { '/foo/bar.txt', 'oil:///foo/', 'bar.txt' },
+      { 'oil:///foo/bar.txt', 'oil:///foo/', 'bar.txt' },
+      { 'oil:///', 'oil:///' },
+      { 'oil-ssh://user@hostname:8888//bar.txt', 'oil-ssh://user@hostname:8888//', 'bar.txt' },
+      { 'oil-ssh://user@hostname:8888//', 'oil-ssh://user@hostname:8888//' },
+    }
+    for _, case in ipairs(cases) do
+      local input, expected, expected_basename = unpack(case)
+      local output, basename = oil.get_buffer_parent_url(input, true)
+      assert.equals(expected, output, string.format('Parent url for path "%s" failed', input))
+      assert.equals(
+        expected_basename,
+        basename,
+        string.format('Basename for path "%s" failed', input)
+      )
+    end
+  end)
+end)