vim-patch:8.2.3671: restarting Insert mode in prompt buffer too often

Problem:    Restarting Insert mode in prompt buffer too often when a callback
            switches windows and comes back. (Sean Dewar)
Solution:   Do not set "restart_edit" when already in Insert mode.
34c20ff85b

As Test_prompt_switch_windows is skipped, implement it in prompt_buffer_spec.

Replace the 50ms term_wait calls with poke_eventloop (test seems to work anyway
without them, so maybe they're not required?)

The new test does include a duplicate screen test that may generate a "screen
test succeeded immediately" warning, but this is done to match the Vim test.

test/functional/legacy/prompt_buffer_spec.lua

@@ -31,6 +31,10 @@ describe('prompt buffer', function()
       func TimerFunc(text)
         call append(line("$") - 1, 'Result: "' . a:text .'"')
       endfunc
+
+      func SwitchWindows()
+        call timer_start(0, {-> execute("wincmd p|wincmd p", "")})
+      endfunc
     ]])
     feed_command("set noshowmode | set laststatus=0")
     feed_command("call setline(1, 'other buffer')")
@@ -167,6 +171,51 @@ describe('prompt buffer', function()
     ]])
   end)
 
+  it('switch windows', function()
+    feed_command("set showmode")
+    feed("i")
+    screen:expect([[
+      cmd: ^                    |
+      ~                        |
+      ~                        |
+      ~                        |
+      [Prompt] [+]             |
+      other buffer             |
+      ~                        |
+      ~                        |
+      ~                        |
+      -- INSERT --             |
+    ]])
+    feed("<C-O>:call SwitchWindows()<CR>")
+    poke_eventloop()
+    screen:expect([[
+      cmd: ^                    |
+      ~                        |
+      ~                        |
+      ~                        |
+      [Prompt] [+]             |
+      other buffer             |
+      ~                        |
+      ~                        |
+      ~                        |
+      -- INSERT --             |
+    ]])
+    feed("<Esc>")
+    poke_eventloop()
+    screen:expect([[
+      cmd:^                     |
+      ~                        |
+      ~                        |
+      ~                        |
+      [Prompt] [+]             |
+      other buffer             |
+      ~                        |
+      ~                        |
+      ~                        |
+                               |
+    ]])
+  end)
+
   it('keeps insert mode after aucmd_restbuf in callback', function()
     source [[
       let s:buf = nvim_create_buf(1, 1)