fix(--headless): do not block on press-enter prompts when no UI

This commit fixes neovim#9358, where emitting multiple messages with 'echo' or
a single one with 'echom' or 'echoerr' would result in a press-enter
prompt that couldn't be dismissed by pressing enter.

This requires adapting a few tests to spawn a UI before testing whether
press-enter prompts are blocking.

It also fixes neovim#11718, as when combined with neovim#15910 it enables making
sure that neovim never blocks and emits messages on startup.

src/nvim/message.c

@@ -1094,6 +1094,10 @@ void wait_return(int redraw)
     return;
   }
 
+  if (headless_mode && !ui_active()) {
+    return;
+  }
+
   /*
    * When inside vgetc(), we can't wait for a typed character at all.
    * With the global command (and some others) we only need one return at

test/functional/api/vim_spec.lua

@@ -333,6 +333,7 @@ describe('API', function()
 
   describe('nvim_command_output', function()
     it('does not induce hit-enter prompt', function()
+      nvim("ui_attach", 80, 20, {})
       -- Induce a hit-enter prompt use nvim_input (non-blocking).
       nvim('command', 'set cmdheight=1')
       nvim('input', [[:echo "hi\nhi2"<CR>]])
@@ -1093,7 +1094,20 @@ describe('API', function()
       eq({mode='n', blocking=false}, nvim("get_mode"))
     end)
 
+    it("during press-enter prompt without UI returns blocking=false", function()
+      eq({mode='n', blocking=false}, nvim("get_mode"))
+      command("echom 'msg1'")
+      command("echom 'msg2'")
+      command("echom 'msg3'")
+      command("echom 'msg4'")
+      command("echom 'msg5'")
+      eq({mode='n', blocking=false}, nvim("get_mode"))
+      nvim("input", ":messages<CR>")
+      eq({mode='n', blocking=false}, nvim("get_mode"))
+    end)
+
     it("during press-enter prompt returns blocking=true", function()
+      nvim("ui_attach", 80, 20, {})
       eq({mode='n', blocking=false}, nvim("get_mode"))
       command("echom 'msg1'")
       command("echom 'msg2'")
@@ -1117,6 +1131,7 @@ describe('API', function()
 
     -- TODO: bug #6247#issuecomment-286403810
     it("batched with input", function()
+      nvim("ui_attach", 80, 20, {})
       eq({mode='n', blocking=false}, nvim("get_mode"))
       command("echom 'msg1'")
       command("echom 'msg2'")