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_autocmd.vim b/src/testdir/test_autocmd.vim
index 639f9eb..1e8711c 100644
--- a/src/testdir/test_autocmd.vim
+++ b/src/testdir/test_autocmd.vim
@@ -194,7 +194,7 @@
     exe 'autocmd BufUnload <buffer> ' . (lastbuf + 1) . 'bwipeout!'
   augroup END
 
-  call assert_fails('edit bb.txt', ['E937:', 'E517:'])
+  call assert_fails('edit bb.txt', 'E937:')
 
   autocmd! test_autocmd_bufunload
   augroup! test_autocmd_bufunload
@@ -1768,7 +1768,7 @@
 func Test_nocatch_wipe_all_buffers()
   " Real nasty autocommand: wipe all buffers on any event.
   au * * bwipe *
-  call assert_fails('next x', ['E94:', 'E517:'])
+  call assert_fails('next x', ['E94:', 'E937:'])
   bwipe
   au!
 endfunc