patch 8.2.4296: Vim9: not all code covered by tests

Problem:    Vim9: not all code covered by tests.
Solution:   Add a few more tests for corner cases.  Fix hang when single quote
            is missing.
diff --git a/src/testdir/test_vim9_cmd.vim b/src/testdir/test_vim9_cmd.vim
index bacf195..51a62e0 100644
--- a/src/testdir/test_vim9_cmd.vim
+++ b/src/testdir/test_vim9_cmd.vim
@@ -1449,7 +1449,7 @@
     ex = v:exception
   endtry
   assert_match('E1121:', ex)
-  unlockvar d.a
+  unlockvar d['a']
   d.a = 7
   assert_equal({a: 7, b: 5}, d)