patch 8.1.2381: not all register related code is covered by tests

Problem:    Not all register related code is covered by tests.
Solution:   Add more test cases. (Yegappan Lakshmanan, closes #5301)

src/testdir/test_marks.vim

@@ -174,3 +174,20 @@ func Test_mark_error()
   call assert_fails('mark xx', 'E488:')
   call assert_fails('mark _', 'E191:')
 endfunc
+
+" Test for :lockmarks when pasting content
+func Test_lockmarks_with_put()
+  new
+  call append(0, repeat(['sky is blue'], 4))
+  normal gg
+  1,2yank r
+  put r
+  normal G
+  lockmarks put r
+  call assert_equal(2, line("'["))
+  call assert_equal(3, line("']"))
+
+  bwipe!
+endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab