test: replace busted with local harness

Replace the busted-based Lua test runner with a repo-local harness.

The new harness runs spec files directly under `nvim -ll`, ships its own
reporter and lightweight `luassert` shim, and keeps the helper/preload
flow used by the functional and unit test suites.

Keep the file boundary model shallow and busted-like by restoring `_G`,
`package.loaded`, `package.preload`, `arg`, and the process environment
between files, without carrying extra reset APIs or custom assertion
machinery.

Update the build and test entrypoints to use the new runner, add
black-box coverage for the harness itself, and drop the bundled
busted/luacheck dependency path.

AI-assisted: Codex
(cherry picked from commit 55f9c2136e)

test/functional/lua/buffer_updates_spec.lua

@@ -8,7 +8,6 @@ local api = n.api
 local fn = n.fn
 local clear = n.clear
 local eq = t.eq
-local fail = t.fail
 local exec_lua = n.exec_lua
 local feed = n.feed
 local expect_events = t.expect_events
@@ -507,7 +506,7 @@ describe('lua: nvim_buf_attach on_bytes', function()
       for _, event in ipairs(events) do
         for _, elem in ipairs(event) do
           if type(elem) == 'number' and elem < 0 then
-            fail(string.format('Received event has negative values'))
+            error('Received event has negative values')
           end
         end
 

test/functional/lua/vim_spec.lua

@@ -834,6 +834,26 @@ describe('lua stdlib', function()
     )
   end)
 
+  it('vim._copy', function()
+    ok(exec_lua([[
+      local inner = { x = 1 }
+      local mt = { tag = true }
+      local a = setmetatable({ inner = inner }, mt)
+      local b = vim._copy(a)
+
+      local c = vim.empty_dict()
+      c.inner = inner
+      local d = vim._copy(c)
+
+      return b ~= a
+        and b.inner == inner
+        and getmetatable(b) == mt
+        and d ~= c
+        and d.inner == inner
+        and not vim.islist(d)
+    ]]))
+  end)
+
   it('vim.pesc', function()
     eq('foo%-bar', exec_lua([[return vim.pesc('foo-bar')]]))
     eq('foo%%%-bar', exec_lua([[return vim.pesc(vim.pesc('foo-bar'))]]))
@@ -1249,7 +1269,11 @@ describe('lua stdlib', function()
     eq(true, exec_lua [[ return vim.deep_equal({a={b=1}}, {a={b=1}}) ]])
     eq(true, exec_lua [[ return vim.deep_equal({a={b={nil}}}, {a={b={}}}) ]])
     eq(true, exec_lua [[ return vim.deep_equal({a=1, [5]=5}, {nil,nil,nil,nil,5,a=1}) ]])
-    eq(true, exec_lua [[ local shared = {}; return vim.deep_equal({ 1, shared, 1, shared }, { 1, {}, 1, {} }) ]])
+    eq(
+      true,
+      exec_lua [[ local shared = {}; return vim.deep_equal({ 1, shared, 1, shared }, { 1, {}, 1, {} }) ]]
+    )
+    -- cyclic table
     eq(true, exec_lua [[ local a,b={},{}; a[1]=a; b[1]=b; return vim.deep_equal(a, b) ]])
     eq(false, exec_lua [[ return vim.deep_equal(1, {nil,nil,nil,nil,5,a=1}) ]])
     eq(false, exec_lua [[ return vim.deep_equal(1, 3) ]])