patch 9.1.0765: No test for patches 6.2.418 and 7.3.489

Problem:  No test for patches 6.2.418 and 7.3.489
Solution: Add a test.  Fix some whitespace problems in test_mapping.vim.
          Document the behavior (zeertzjq).

closes: #15815

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index 2b392c5..790efaa 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -3920,7 +3920,7 @@
   set cpo-=*
 endfunc
 
-func Test_cd_bslsh_completion_windows()
+func Test_cd_bslash_completion_windows()
   CheckMSWindows
   let save_shellslash = &shellslash
   set noshellslash