patch 9.0.1315: escaping for completion of map command not properly tested

Problem:    Escaping for completion of map command not properly tested.
Solution:   Add a few test cases. (closes #12009)
diff --git a/src/testdir/test_cpoptions.vim b/src/testdir/test_cpoptions.vim
index a5822aa..6ff8301 100644
--- a/src/testdir/test_cpoptions.vim
+++ b/src/testdir/test_cpoptions.vim
@@ -67,15 +67,20 @@
 func Test_cpo_B()
   let save_cpo = &cpo
   new
+  imap <buffer> x<Bslash>k Test
   set cpo-=B
   iabbr <buffer> abc ab\<BS>d
   exe "normal iabc "
   call assert_equal('ab<BS>d ', getline(1))
+  call feedkeys(":imap <buffer> x\<C-A>\<C-B>\"\<CR>", 'tx')
+  call assert_equal('"imap <buffer> x\\k', @:)
   %d
   set cpo+=B
   iabbr <buffer> abc ab\<BS>d
   exe "normal iabc "
   call assert_equal('abd ', getline(1))
+  call feedkeys(":imap <buffer> x\<C-A>\<C-B>\"\<CR>", 'tx')
+  call assert_equal('"imap <buffer> x\k', @:)
   close!
   let &cpo = save_cpo
 endfunc
@@ -192,7 +197,8 @@
   set cpo+=f
   read test_cpoptions.vim
   call assert_equal('test_cpoptions.vim', @%)
-  close!
+
+  bwipe!
   let &cpo = save_cpo
 endfunc
 
@@ -438,7 +444,8 @@
   set cpo+=P
   write >> XfileCpoP
   call assert_equal('XfileCpoP', @%)
-  close!
+
+  bwipe!
   let &cpo = save_cpo
 endfunc