patch 8.2.1937: Vim9: test for confirm modifier fails in some situations

Problem:    Vim9: test for confirm modifier fails in some situations.
Solution:   Add a short wait.  Handle failure better.
diff --git a/src/testdir/test_vim9_cmd.vim b/src/testdir/test_vim9_cmd.vim
index a355af1..a87d9b2 100644
--- a/src/testdir/test_vim9_cmd.vim
+++ b/src/testdir/test_vim9_cmd.vim
@@ -392,10 +392,14 @@
   call term_sendkeys(buf, ":call Getout()\n")
   call WaitForAssert({-> assert_match('(Y)es, \[N\]o: ', term_getline(buf, 8))}, 1000)
   call term_sendkeys(buf, "y")
+  call WaitForAssert({-> assert_match('(Y)es, \[N\]o: ', term_getline(buf, 8))}, 1000)
+  call term_sendkeys(buf, "\<CR>")
+  call TermWait(buf)
   call StopVimInTerminal(buf)
 
   call assert_equal(['changed'], readfile('Xfile'))
   call delete('Xfile')
+  call delete('.Xfile.swp')  " in case Vim was killed
   call delete('Xconfirmscript')
 endfunc