patch 8.2.3982: some lines of code not covered by tests

Problem:    Some lines of code not covered by tests.
Solution:   Add a few more test cases. (Dominique Pellé, closes #9453)
diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index df9e8ec..2ee1a93 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -956,6 +956,8 @@
 
   assert_equal("yes", expandcmd("`={a: 'yes'}['a']`"))
   expandcmd('')->assert_equal('')
+
+  CheckDefAndScriptFailure(['expandcmd([1])'], ['E1013: Argument 1: type mismatch, expected string but got list<number>', 'E1174: String required for argument 1'])
 enddef
 
 def Test_extend_arg_types()