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()