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_vim9_func.vim b/src/testdir/test_vim9_func.vim
index 919af74..4ec0f6f 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -1467,14 +1467,14 @@
   g:did_it = 'yes'
 enddef
 
-"func UserError()
-"  silent! invalid
-"endfunc
-"
-"def SilentlyUserError()
-"  UserError()
-"  g:did_it = 'yes'
-"enddef
+func UserError()
+  silent! invalid
+endfunc
+
+def SilentlyUserError()
+  UserError()
+  g:did_it = 'yes'
+enddef
 
 " This can't be a :def function, because the assert would not be reached.
 " And this must not be inside a try/endtry.
@@ -1483,10 +1483,9 @@
   call SilentlyError()
   call assert_equal('yes', g:did_it)
 
-"  this doesn't work yet
-"  let g:did_it = 'no'
-"  call SilentlyUserError()
-"  call assert_equal('yes', g:did_it)
+  let g:did_it = 'no'
+  call SilentlyUserError()
+  call assert_equal('yes', g:did_it)
 
   unlet g:did_it
 endfunc