patch 8.2.0448: various functions not properly tested

Problem:    Various functions not properly tested.
Solution:   Add more tests, especially for failures. (Yegappan Lakshmanan,
            closes #5843)
diff --git a/src/testdir/test_file_perm.vim b/src/testdir/test_file_perm.vim
index 3950168..1cb09e8 100644
--- a/src/testdir/test_file_perm.vim
+++ b/src/testdir/test_file_perm.vim
@@ -21,4 +21,10 @@
 
   call assert_equal(1, setfperm('Xtest', 'rwx------'))
   call delete('Xtest')
+
+  call assert_fails("call setfperm(['Xfile'], 'rw-rw-rw-')", 'E730:')
+  call assert_fails("call setfperm('Xfile', [])", 'E730:')
+  call assert_fails("call setfperm('Xfile', 'rwxrwxrwxrw')", 'E475:')
 endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab