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/util_spec.lua

@@ -0,0 +1,29 @@
+local util = require('oil.util')
+describe('util', function()
+  it('url_escape', function()
+    local cases = {
+      { 'foobar', 'foobar' },
+      { 'foo bar', 'foo%20bar' },
+      { '/foo/bar', '%2Ffoo%2Fbar' },
+    }
+    for _, case in ipairs(cases) do
+      local input, expected = unpack(case)
+      local output = util.url_escape(input)
+      assert.equals(expected, output)
+    end
+  end)
+
+  it('url_unescape', function()
+    local cases = {
+      { 'foobar', 'foobar' },
+      { 'foo%20bar', 'foo bar' },
+      { '%2Ffoo%2Fbar', '/foo/bar' },
+      { 'foo%%bar', 'foo%%bar' },
+    }
+    for _, case in ipairs(cases) do
+      local input, expected = unpack(case)
+      local output = util.url_unescape(input)
+      assert.equals(expected, output)
+    end
+  end)
+end)