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_marks.vim b/src/testdir/test_marks.vim
index 1d1f363..15be080 100644
--- a/src/testdir/test_marks.vim
+++ b/src/testdir/test_marks.vim
@@ -90,6 +90,8 @@
   call setpos('.', [0, 1, -1, 0])
   call assert_equal([2, 2], [line('.'), col('.')])
 
+  call assert_fails("call setpos('ab', [0, 1, 1, 0])", 'E474:')
+
   bwipe!
   call win_gotoid(twowin)
   bwipe!