tests: Add a test for matching by two well-known names

The expected result is that because TEST_CONN_SERVICE owns
ALREADY_OWNED_NAME but not (yet) OWNED_LATER_NAME, the signal will be
delivered to the subscriber for the former but not the latter.
Before #3268 was fixed, it was incorrectly delivered to both.

Reproduces: https://gitlab.gnome.org/GNOME/glib/-/issues/3268 (partially)
Signed-off-by: Simon McVittie <smcv@collabora.com>

gio/tests/gdbus-subscribe.c

@@ -440,6 +440,19 @@ static const TestPlan plan_limit_by_well_known_name =
         .iface = EXAMPLE_INTERFACE,
       },
     },
+    {
+      /* When the service sends a signal with the name it already owns,
+       * it should get through */
+      .action = TEST_ACTION_EMIT_SIGNAL,
+      .u.signal = {
+        .sender = TEST_CONN_SERVICE,
+        .path = EXAMPLE_PATH,
+        .iface = EXAMPLE_INTERFACE,
+        .member = FOO_SIGNAL,
+        .received_by_conn = 1,
+        .received_by_proxy = 1
+      },
+    },
     {
       /* Service claims another name */
       .action = TEST_ACTION_OWN_NAME,