test(tui_spec): avoid dangling Nvim processes on test failure (#35900)

Also don't wait indefinitely for all Nvim processes to exit.

test/functional/terminal/tui_spec.lua

@@ -3820,6 +3820,10 @@ describe('TUI client', function()
   it('connects to remote instance (with its own TUI)', function()
     local server_super = n.new_session(false)
     local client_super = n.new_session(true)
+    finally(function()
+      server_super:close()
+      client_super:close()
+    end)
 
     set_session(server_super)
     local server_pipe = new_pipename()
@@ -3894,14 +3898,15 @@ describe('TUI client', function()
     screen_client:expect({ any = 'GUI Running: 0' })
 
     feed_data(':q!\n')
-
-    server_super:close()
-    client_super:close()
   end)
 
   it('connects to remote instance (--headless)', function()
     local server = n.new_session(false)
     local client_super = n.new_session(true, { env = { NVIM_LOG_FILE = testlog } })
+    finally(function()
+      client_super:close()
+      server:close()
+    end)
 
     set_session(server)
     local server_pipe = api.nvim_get_vvar('servername')
@@ -3971,8 +3976,6 @@ describe('TUI client', function()
     feed_data(':echo "GUI Running: " .. has("gui_running")\013')
     screen_client:expect({ any = 'GUI Running: 0' })
 
-    client_super:close()
-    server:close()
     if is_os('mac') then
       assert_log('uv_tty_set_mode failed: Unknown system error %-102', testlog)
     end

test/functional/testnvim.lua

@@ -519,15 +519,26 @@ function M.new_session(keep, ...)
   n_processes = n_processes + 1
 
   local new_session = Session.new(proc)
+  -- Make it possible to check whether two sessions are from the same test.
   new_session.data = { test_id = test_id }
   return new_session
 end
 
 busted.subscribe({ 'suite', 'end' }, function()
   M.check_close(true)
-  while n_processes > 0 do
+  local timed_out = false
+  local timer = assert(vim.uv.new_timer())
+  timer:start(10000, 0, function()
+    timed_out = true
+  end)
+  while n_processes > 0 and not timed_out do
     uv.run('once')
   end
+  timer:close()
+  if timed_out then
+    print(('warning: %d dangling Nvim processes'):format(n_processes))
+    io.stdout:flush()
+  end
 end)
 
 --- Starts a (non-RPC, `--headless --listen "Tx"`) Nvim process, waits for exit, and returns result.