test: flaky terminal channel exitcode check #39580

Problem:
The `nvim_get_chan_info()` terminal channel test used `shell-test INTERACT` to verify that `jobstop()` reports an unhandled `SIGHUP` as exit code `129`.

`INTERACT` reads from stdin with `fgets()`, so closing the PTY could race with `SIGHUP` delivery. If `fgets()` observed EOF first, `shell-test` exited normally with code `0`, causing intermittent failures on slower sanitizer builds.

Solution:
Add a `shell-test HOLD` mode that prints a readiness prompt and then waits without reading stdin. Use it for the `SIGHUP` assertion so PTY EOF cannot make the helper exit normally before the signal path is observed.

test/functional/api/vim_spec.lua

@@ -2977,9 +2977,10 @@ describe('API', function()
 
       -- :terminal with args + stopped process (shell-test).
       command('enew')
-      argv = { n.testprg('shell-test'), 'INTERACT' }
+      -- Use a process that doesn't read stdin, so PTY EOF can't race SIGHUP.
+      argv = { n.testprg('shell-test'), 'HOLD' }
       fn.jobstart(argv, { term = true })
-      screen:expect({ any = { vim.pesc('interact $') } })
+      screen:expect({ any = { vim.pesc('holding $') } })
       eq(1, eval('jobstop(&channel)'))
       eval('jobwait([&channel], 1000)') -- Wait.
       local expected3 = term_channel_info(5, 3, argv)