test: disable at_exit LSan check for dbus.service

test: disable at_exit LSan check for dbus.service

When running integration tests under sanitizers D-Bus fails to
shutdown cleanly, causing unnecessary noise in the logs:

```
dbus-daemon[272]: ==272==LeakSanitizer has encountered a fatal error.
dbus-daemon[272]: ==272==HINT: For debugging, try setting environment variable LSAN_OPTIONS=verbosity=1:log_threads=1
dbus-daemon[272]: ==272==HINT: LeakSanitizer does not work under ptrace (strace, gdb, etc)
```

Since we're not "sanitizing" D-Bus anyway let's disable LSan's at_exit
check for the dbus.service to get rid of this error.