t5710: arrange to delete the client before cloning

t5710: arrange to delete the client before cloning

If `test_when_finished "rm -rf client"` is run after we clone, it
will not run if the clone failed, so the "client" directory might
not be removed at the end of the test.

`git clone` does try to remove the directory when it fails, but
let's be safe and try to protect against possibly weird clone
failures by moving `test_when_finished "rm -rf client"` before
the clone. It just makes more sense this way around.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>