patch 8.2.1919: assert_fails() setting emsg_silent changes normal execution

Problem:    Assert_fails() setting emsg_silent changes normal execution.
Solution:   Use a separate flag in_assert_fails.
diff --git a/src/testdir/test_popup.vim b/src/testdir/test_popup.vim
index 381a2c1..28481b3 100644
--- a/src/testdir/test_popup.vim
+++ b/src/testdir/test_popup.vim
@@ -344,7 +344,7 @@
   /^one
   call assert_fails('call feedkeys("A\<C-X>\<C-U>\<C-N>\<Esc>", "x")', 'E578:')
   call assert_equal(winid, win_getid())
-  call assert_equal('oneDEF', getline(1))
+  call assert_equal('onedef', getline(1))
   q!
 endfunc