paste: workaround typeahead race

Workaround this failure:

    [  ERROR   ] test/functional/terminal/tui_spec.lua @ 192: TUI paste: exactly 64 bytes
    test/functional/helpers.lua:403:
    retry() attempts: 478
    test/functional/terminal/tui_spec.lua:201: Expected objects to be the same.
    Passed in:
    (table: 0x47cd77e8) {
     *[1] = 'zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz endz' }
    Expected:
    (table: 0x47cd7830) {
     *[1] = 'zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz end' }

This happens because `curwin->w_cursor.col` is sometimes decremented at
the end of `do_put`... because the editor is in Normal-mode instead of
the expected Insert-mode.

Caused by "typeahead race" (#10826): there may be queued input in the
main thread not yet processed, thus the editor mode (`State` global)
will be "wrong" during paste. Example: input "i" followed immediately by
a paste sequence:

    i<start-paste>...<stop-paste>
    ^
     "i" does not get processed in time, so the editor is in
     Normal-mode instead of Insert-mode while handling the paste.

Attempted workarounds:
- vim.api.nvim_feedkeys('','x',false) in vim._paste()
- exec_normal() in tinput_wait_enqueue()
- LOOP_PROCESS_EVENTS(&main_loop,…,0) in tinput_wait_enqueue()

ref #10826

test/functional/terminal/tui_spec.lua

@@ -191,8 +191,14 @@ describe('TUI', function()
 
   it('paste: exactly 64 bytes #10311', function()
     local expected = string.rep('z', 64)
+    feed_data('i')
+    -- Wait for Insert-mode (avoid "typeahead race" #10826).
+    retry(nil, nil, function()
+      local _, m = child_session:request('nvim_get_mode')
+      eq('i', m.mode)
+    end)
     -- "bracketed paste"
-    feed_data('i\027[200~'..expected..'\027[201~')
+    feed_data('\027[200~'..expected..'\027[201~')
     feed_data(' end')
     expected = expected..' end'
     retry(nil, nil, function()
@@ -217,8 +223,14 @@ describe('TUI', function()
     for i = 1, 3000 do
       t[i] = 'item ' .. tostring(i)
     end
+    feed_data('i')
+    -- Wait for Insert-mode (avoid "typeahead race" #10826).
+    retry(nil, nil, function()
+      local _, m = child_session:request('nvim_get_mode')
+      eq('i', m.mode)
+    end)
     -- "bracketed paste"
-    feed_data('i\027[200~'..table.concat(t, '\n')..'\027[201~')
+    feed_data('\027[200~'..table.concat(t, '\n')..'\027[201~')
     retry(nil, nil, function()
       local _, buflines = child_session:request(
         'nvim_buf_get_lines', 0, 0, -1, false)