Increase time allowed for dbus-installed-tests

The "as-root" run of dbus-installed-test failed as it exceeded the time
limit by 0.04s. Increase the time limit from 15 mins to 20 mins to avoid
this happening.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>

test-cases/dbus-installed-tests.yaml

@@ -34,7 +34,7 @@ run:
     - cd apertis-tests
     - "# Execute the following commands:"
     - common/run-test-in-systemd --user=user --name=as-user --timeout=15min dbus/installed-tests.sh
-    - common/run-test-in-systemd --name=as-root --timeout=15min dbus/installed-tests.sh
+    - common/run-test-in-systemd --name=as-root --timeout=20min dbus/installed-tests.sh
 
 parse:
   pattern: 'TEST_RESULT:(?P<result>\w+):(?P<test_case_id>[^:]+):'