test-ls.sh: handle Cygwin's coupling of timestamp/permission modifications.

In Cygwin, the file's modification date is updated when its
permissions are modified. To make the tests compatible with Cygwin,
create the files first, set their permissions, then set the date.

Signed-off-by: Patrick Rouleau <prouleau72@gmail.com>
[rlb@defaultvalue.org: adjust commit message.]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>