tools: Supress memory leak in g_test_dbus_down()

This happens in travis:

```
PASS: test-webservice 11 /web-service/logout
PASS: test-webservice 12 /web-service/handshake-and-auth/rfc6455
PASS: test-webservice 13 /web-service/handshake-and-auth/hixie76
PASS: test-webservice 14 /web-service/echo-message/rfc6455
PASS: test-cgroupmonitor 4 /cgroup-monitor/zero-limits
PASS: test-webservice 15 /web-service/echo-message/hixie76
==26526== 13,136 (24 direct, 13,112 indirect) bytes in 1 blocks are definitely lost in loss record 1,383 of 1,384
==26526==    at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==26526==    by 0x5449610: g_malloc (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4000.0)
==26526==    by 0x545F22D: g_slice_alloc (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4000.0)
==26526==    by 0x545F76D: g_slice_alloc0 (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4000.0)
==26526==    by 0x4F0E92C: ??? (in /usr/lib/x86_64-linux-gnu/libgio-2.0.so.0.4000.0)
==26526==    by 0x4F0E9B6: ??? (in /usr/lib/x86_64-linux-gnu/libgio-2.0.so.0.4000.0)
==26526==    by 0x4EFACEE: ??? (in /usr/lib/x86_64-linux-gnu/libgio-2.0.so.0.4000.0)
==26526==    by 0x51C0ACF: g_object_run_dispose (in /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0.4000.0)
==26526==    by 0x4F1D298: ??? (in /usr/lib/x86_64-linux-gnu/libgio-2.0.so.0.4000.0)
==26526==    by 0x5443CE4: g_main_context_dispatch (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4000.0)
==26526==    by 0x5444047: ??? (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4000.0)
==26526==    by 0x5444309: g_main_loop_run (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4000.0)
==26526==    by 0x4F1DC60: g_test_dbus_down (in /usr/lib/x86_64-linux-gnu/libgio-2.0.so.0.4000.0)
==26526==    by 0x406EF1: teardown (test-cgroupmonitor.c:259)
==26526==    by 0x5467AE4: ??? (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4000.0)
==26526==    by 0x5467C55: ??? (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4000.0)
==26526== 
{
   <insert_a_suppression_name_here>
   Memcheck:Leak
   match-leak-kinds: definite
   fun:malloc
   fun:g_malloc
   fun:g_slice_alloc
   fun:g_slice_alloc0
   obj:/usr/lib/x86_64-linux-gnu/libgio-2.0.so.0.4000.0
   obj:/usr/lib/x86_64-linux-gnu/libgio-2.0.so.0.4000.0
   obj:/usr/lib/x86_64-linux-gnu/libgio-2.0.so.0.4000.0
   fun:g_object_run_dispose
   obj:/usr/lib/x86_64-linux-gnu/libgio-2.0.so.0.4000.0
   fun:g_main_context_dispatch
   obj:/lib/x86_64-linux-gnu/libglib-2.0.so.0.4000.0
   fun:g_main_loop_run
   fun:g_test_dbus_down
   fun:teardown
   obj:/lib/x86_64-linux-gnu/libglib-2.0.so.0.4000.0
   obj:/lib/x86_64-linux-gnu/libglib-2.0.so.0.4000.0
}
ERROR: test-cgroupmonitor process failed: 33
PASS: test-webservice 16 /web-service/echo-message/large
WebSocket-Message: connection unexpectedly closed by peer
PASS: test-webservice 17 /web-service/bad-origin/rfc6455
````