test: run Lua harness with nvim -l

Problem:
The Lua test harness still ran through standalone -ll mode, so tests
depended on the low-level Lua path instead of the regular Nvim Lua
environment. That also meant os.exit() coverage had to carry an ASAN
workaround because Lua's raw process exit skipped Nvim teardown and let
LeakSanitizer interfere with the observed exit code.

Solution:
Run the harness and related fixtures with nvim -l. Patch os.exit() in
the main Lua state to exit through getout(), so scripts observe normal
Nvim shutdown while standalone -ll remains available for generator-style
scripts. As a consequence, the startup test can assert os.exit() without
disabling leak detection.

AI-assisted: Codex

test/functional/api/ui_spec.lua

@@ -94,6 +94,13 @@ describe('nvim_ui_send', function()
     clear()
   end)
 
+  local function close_pipe(pipe)
+    if not pipe:is_closing() then
+      pipe:read_stop()
+      pipe:close()
+    end
+  end
+
   it('works with stdout_tty', function()
     local fds = assert(uv.pipe())
 
@@ -110,6 +117,10 @@ describe('nvim_ui_send', function()
 
     local screen = Screen.new(50, 10, { stdout_tty = true })
     screen:set_stdout(fds.write)
+    finally(function()
+      screen:detach()
+      close_pipe(read_pipe)
+    end)
 
     api.nvim_ui_send('Hello world')
 
@@ -140,6 +151,10 @@ describe('nvim_ui_send', function()
 
     local screen = Screen.new(50, 10)
     screen:set_stdout(fds.write)
+    finally(function()
+      screen:detach()
+      close_pipe(read_pipe)
+    end)
 
     api.nvim_ui_send('Hello world')