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.

src/testdir/test_vim9_assign.vim

@@ -1993,6 +1993,12 @@ def Test_unlet()
   assert_false(exists('s:somevar'))
   unlet! s:somevar
 
+  if 0
+    unlet g:does_not_exist
+  endif
+
+  v9.CheckDefExecFailure(['unlet v:notfound.key'], 'E1001:')
+
   v9.CheckDefExecFailure([
     'var dd = 111',
     'unlet dd',