patch 8.2.0401: not enough test coverage for evalvars.c

Problem:    Not enough test coverage for evalvars.c.
Solution:   Add more tests. (Yegappan Lakshmanan, closes #5804)
diff --git a/src/testdir/test_vimscript.vim b/src/testdir/test_vimscript.vim
index cac3be5..a9d246b 100644
--- a/src/testdir/test_vimscript.vim
+++ b/src/testdir/test_vimscript.vim
@@ -1623,6 +1623,9 @@
     endfunc
     delfunc! Xtest
     delfunc! Xtest
+
+    " Try deleting the current function
+    call assert_fails('delfunc Test_delfunction_force', 'E131:')
 endfunc
 
 " Test using bang after user command				    {{{1