test: bump the client-side timeout in sd-bus as well

Since c78d18215b D-Bus services now have 60s to start, but the client
side (sd-bus) still waits only for 25s before giving up:

```
[  226.196380] testsuite-71.sh[556]: + assert_in 'Static hostname: H' ''
[  226.332965] testsuite-71.sh[576]: + set +ex
[  226.332965] testsuite-71.sh[576]: FAIL: 'Static hostname: H' not found in:
[  228.910782] sh[577]: + systemctl poweroff --no-block
[  232.255584] hostnamectl[565]: Failed to query system properties: Connection timed out
[  236.827514] systemd[1]: end.service: Consumed 2.131s CPU time.
[  237.476969] dbus-daemon[566]: [system] Successfully activated service 'org.freedesktop.hostname1'
[  237.516308] systemd[1]: system-modprobe.slice: Consumed 1.533s CPU time.
[  237.794635] systemd[1]: testsuite-71.service: Main process exited, code=exited, status=1/FAILURE
[  237.818469] systemd[1]: testsuite-71.service: Failed with result 'exit-code'.
[  237.931415] systemd[1]: Failed to start testsuite-71.service.
[  238.000833] systemd[1]: testsuite-71.service: Consumed 5.651s CPU time.
[  238.181030] systemd[1]: Reached target testsuite.target.
```

Let's override the timeout in sd-bus as well to mitigate this.

Follow-up to c78d18215b.

test/test-functions

@@ -1943,6 +1943,9 @@ EOF
     <limit name="service_start_timeout">60000</limit>
 </busconfig>
 EOF
+        # Bump the client-side timeout in sd-bus as well
+        mkdir -p "$initdir/etc/systemd/system.conf.d"
+        echo -e '[Manager]\nDefaultEnvironment=SYSTEMD_BUS_TIMEOUT=60' >"$initdir/etc/systemd/system.conf.d/bus-timeout.conf"
     fi
 }