patch 8.2.0470: Test_confirm_cmd_cancel() can fail on a slow system

Problem:    Test_confirm_cmd_cancel() can fail on a slow system.
Solution:   Use WaitForAssert(). (Ozaki Kiichi, closes #5861)
diff --git a/src/testdir/test_excmd.vim b/src/testdir/test_excmd.vim
index d5588d6..2d4d520 100644
--- a/src/testdir/test_excmd.vim
+++ b/src/testdir/test_excmd.vim
@@ -259,7 +259,7 @@
   call WaitForAssert({-> assert_match('^\[Y\]es, (N)o, (C)ancel: *$',
         \ term_getline(buf, 20))}, 1000)
   call term_sendkeys(buf, "C")
-  call term_wait(buf, 50)
+  call WaitForAssert({-> assert_equal('', term_getline(buf, 20))}, 1000)
   call term_sendkeys(buf, ":confirm close\n")
   call WaitForAssert({-> assert_match('^\[Y\]es, (N)o, (C)ancel: *$',
         \ term_getline(buf, 20))}, 1000)