t1509: fix failing "root work tree" test due to owner-check

When 8959555cee (setup_git_directory(): add an owner check for the
top-level directory, 2022-03-02) tightened security surrounding
directory ownership, it neglected to adjust t1509-root-work-tree.sh to
take the new restriction into account. As a result, since the root
directory `/` is typically not owned by the user running the test
(indeed, t1509 refuses to run as `root`), the ownership check added
by 8959555cee kicks in and causes the test to fail:

fatal: detected dubious ownership in repository at '/'
To add an exception for this directory, call:

git config --global --add safe.directory /

This problem went unnoticed for so long because t1509 is rarely run
since it requires setting up a `chroot` environment or a sacrificial
virtual machine in which `/` can be made writable and polluted by any
user.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>