patch 8.2.1357: Vim9: cannot assign to / register

Problem:    Vim9: cannot assign to / register.
Solution:   Adjust check for assignment.
diff --git a/src/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index ef8797f..584126f 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -1439,6 +1439,13 @@
 
   normal axyz
   assert_equal("xyz", @.)
+  call CheckDefFailure(["@. = 'yes'"], 'E354:')
+
+  @/ = 'slash'
+  assert_equal('slash', @/)
+
+  @= = 'equal'
+  assert_equal('equal', @=)
 enddef
 
 def Test_expr7_namespace()
diff --git a/src/testdir/test_vim9_script.vim b/src/testdir/test_vim9_script.vim
index 5ed9a3d..5c2db2e 100644
--- a/src/testdir/test_vim9_script.vim
+++ b/src/testdir/test_vim9_script.vim
@@ -470,8 +470,9 @@
   call CheckDefFailure(['let $VAR = 5'], 'E1016: Cannot declare an environment variable:')
   call CheckScriptFailure(['vim9script', 'let $ENV = "xxx"'], 'E1016:')
 
-  call CheckDefFailure(['let @~ = 5'], 'E354:')
+  call CheckDefFailure(['let @~ = 5'], 'E1066:')
   call CheckDefFailure(['let @a = 5'], 'E1066:')
+  call CheckDefFailure(['let @/ = "x"'], 'E1066:')
   call CheckScriptFailure(['vim9script', 'let @a = "abc"'], 'E1066:')
 
   call CheckDefFailure(['let g:var = 5'], 'E1016: Cannot declare a global variable:')