patch 8.2.5025: Vim9: a few lines not covered by tests

Problem:    Vim9: a few lines not covered by tests.
Solution:   Add a few tests.
diff --git a/src/testdir/test_vim9_assign.vim b/src/testdir/test_vim9_assign.vim
index a13e370..b0ce398 100644
--- a/src/testdir/test_vim9_assign.vim
+++ b/src/testdir/test_vim9_assign.vim
@@ -213,6 +213,8 @@
   v9.CheckDefFailure(['var s:var = 123'], 'E1101:')
   v9.CheckDefFailure(['var s:var: number'], 'E1101:')
 
+  v9.CheckDefAndScriptFailure(['var $VAR: number'], ['E1016:', 'E475:'])
+
   lines =<< trim END
     vim9script
     def SomeFunc()