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_terminal.vim b/src/testdir/test_terminal.vim
index 676d864..829706c 100644
--- a/src/testdir/test_terminal.vim
+++ b/src/testdir/test_terminal.vim
@@ -65,7 +65,7 @@
 
   setlocal modifiable
   exe "normal Axxx\<Esc>"
-  call assert_fails(buf . 'bwipe', ['E89:', 'E517:'])
+  call assert_fails(buf . 'bwipe', 'E89:')
   undo
 
   exe buf . 'bwipe'
@@ -89,7 +89,7 @@
 
 func Test_terminal_wipe_buffer()
   let buf = Run_shell_in_terminal({})
-  call assert_fails(buf . 'bwipe', ['E89:', 'E517:'])
+  call assert_fails(buf . 'bwipe', 'E89:')
   exe buf . 'bwipe!'
   call WaitForAssert({-> assert_equal('dead', job_status(g:job))})
   call assert_equal("", bufname(buf))
@@ -648,7 +648,7 @@
 
 func Test_terminal_list_args()
   let buf = term_start([&shell, &shellcmdflag, 'echo "123"'])
-  call assert_fails(buf . 'bwipe', ['E89:', 'E517:'])
+  call assert_fails(buf . 'bwipe', 'E89:')
   exe buf . 'bwipe!'
   call assert_equal("", bufname(buf))
 endfunction