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.

oil.nvim-scm-1.rockspec

@@ -16,6 +16,15 @@ dependencies = {
   'lua >= 5.1',
 }
 
+test_dependencies = {
+  'nlua',
+  'busted >= 2.1.1',
+}
+
+test = {
+  type = 'busted',
+}
+
 build = {
   type = 'builtin',
 }