maintenance: fix test t7900-maintenance.sh

Commit b681b191 introduced the support of systemd timers for git maintenance. A test is leveraging the `systemd-analyze verify` utility to verify the correctness of the systemd unit files generated by git. But on some systems, although the `systemd-analyze` tool is installed and supports the `verify` subcommand, it fails with some permission errors. So, instead of only checking if the `verify` subcommand exists, a more reliable way of detecting whether `systemd-analyze verify` can be used is to try to use it. The SYSTEMD_ANALYZE prerequisite is now trying to run `systemd-analyze verify` on a systemd unit file which is shipped by systemd itself. We can reasonably think that, on systemd hosts, this file is present and valid. Signed-off-by: Lénaïc Huard <lenaic@lhuard.fr> Signed-off-by: Junio C Hamano <gitster@pobox.com>