cmd: Add test for substring handling in 'magic' variable expansion.

programs/cmd/tests/test_builtins.cmd

@@ -622,7 +622,13 @@ echo %WINE_VAR:~2,-1%
 echo %WINE_VAR:~2,-3%
 echo '%WINE_VAR:~-2,-4%'
 echo %WINE_VAR:~-3,-2%
+echo %WINE_VAR:~4,4%
 set WINE_VAR=
+mkdir dummydir
+cd dummydir
+echo %CD:~-6,6%
+cd ..
+rmdir dummydir
 
 echo ------------ Testing variable substitution ------------
 echo --- in FOR variables

programs/cmd/tests/test_builtins.cmd.exp

@@ -532,6 +532,8 @@ ert@or_broken@qwerty
 e@or_broken@qwerty
 ''@or_broken@'qwerty'
 r@or_broken@qwerty
+ty
+@todo_wine@mmydir
 ------------ Testing variable substitution ------------
 --- in FOR variables
 "A B"