tools: fix signal type in service test

The signal definition is:

<signal name="StateChanged">
  <arg name="old" type="i" />
  <arg name="new" type="i" />
  <arg name="reason" type="u" />
</signal>

So the first two arguments need to be adjusted.

Without this change, the test was failing for me:

(/build/source/tools/tests/.libs/lt-test-stub:77030): GLib-GObject-CRITICAL **: 15:05:35.276: ../gobject/gsignal.c:3167: value for 'gint' parameter 0 for signal "state-changed" is of type 'guint'