tests,dbus: append pid to abstract socket name

So that different concurrent runs of this same test don't clash with
each other:

    TEST: test-service-generic... (pid=11124)
      /MM/Service/Generic/enable-disable:                                  Activating service name='org.freedesktop.ModemManager1'
    Successfully activated service 'org.freedesktop.ModemManager1'
    ** (plugins/.libs/lt-test-service-generic>:11124): ERROR **: Cannot bind socket: Error binding to address: Address already in use
    cleaning up pid 11144
    FAIL

plugins/generic/tests/test-service-generic.c

@@ -10,10 +10,12 @@
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details:
  *
- * Copyright (C) 2013 Aleksander Morgado <aleksander@gnu.org>
+ * Copyright (C) 2016 Aleksander Morgado <aleksander@gnu.org>
  */
 
+#include <sys/types.h>
 #include <unistd.h>
+
 #include <glib.h>
 #include <glib-object.h>
 
@@ -31,10 +33,12 @@ test_enable_disable (TestFixture *fixture)
     MMObject *obj;
     MMModem *modem;
     TestPortContext *port0;
-    const gchar *ports [] = {
-        "abstract:port0",
-        NULL
-    };
+    gchar *ports [] = { NULL, NULL };
+
+    /* Create port name, and add process ID so that multiple runs of this test
+     * in the same system don't clash with each other */
+	ports[0] = g_strdup_printf ("abstract:port0:%ld", (glong) getpid ());
+    g_debug ("test service generic: using abstract port at '%s'", ports[0]);
 
     /* Setup new port context */
     port0 = test_port_context_new (ports[0]);
@@ -69,6 +73,8 @@ test_enable_disable (TestFixture *fixture)
     /* Stop port context */
     test_port_context_stop (port0);
     test_port_context_free (port0);
+
+    g_free (ports[0]);
 }
 
 /*****************************************************************************/