patch 8.2.2643: various code not covered by tests

Problem:    Various code not covered by tests.
Solution:   Add a few more test. (Yegappan Lakshmanan, closes #7995)
diff --git a/src/testdir/test_functions.vim b/src/testdir/test_functions.vim
index ca57c30..8ee3679 100644
--- a/src/testdir/test_functions.vim
+++ b/src/testdir/test_functions.vim
@@ -1431,7 +1431,7 @@
   call assert_equal('item1 item2 item3', c)
 
   " Test for using special characters as default input
-  call feedkeys(":let c = input('name? ', \"x\<BS>y\")\<CR>\<CR>", 'xt')
+  call feedkeys(":let c = input('name? ', \"x\\<BS>y\")\<CR>\<CR>", 'xt')
   call assert_equal('y', c)
 
   " Test for using <CR> as default input