patch 8.2.0522: several errors are not tested for

Problem:    Several errors are not tested for.
Solution:   Add tests. (Yegappan Lakshmanan, closes #5892)
diff --git a/src/testdir/test_user_func.vim b/src/testdir/test_user_func.vim
index 433a65b..c8f8d07 100644
--- a/src/testdir/test_user_func.vim
+++ b/src/testdir/test_user_func.vim
@@ -169,3 +169,10 @@
 func Test_failed_call_in_try()
   try | call UnknownFunc() | catch | endtry
 endfunc
+
+" Test for listing user-defined functions
+func Test_function_list()
+  call assert_fails("function Xabc", 'E123:')
+endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab