tests: use sd_bus_flush_close_unref instead of sd_bus_unref in test-bus-cleanup

Fixes:
$ make valgrind-tests TESTS=test-bus-cleanup
==6363== 9 bytes in 1 blocks are possibly lost in loss record 1 of 28
==6363==    at 0x4C2BBCF: malloc (vg_replace_malloc.c:299)
==6363==    by 0x197D12: hexmem (hexdecoct.c:79)
==6363==    by 0x183083: bus_socket_start_auth_client (bus-socket.c:639)
==6363==    by 0x1832A0: bus_socket_start_auth (bus-socket.c:678)
==6363==    by 0x183438: bus_socket_connect (bus-socket.c:705)
==6363==    by 0x14B0F2: bus_start_address (sd-bus.c:1053)
==6363==    by 0x14B592: sd_bus_start (sd-bus.c:1134)
==6363==    by 0x14B95E: sd_bus_open_system (sd-bus.c:1235)
==6363==    by 0x1127E2: test_bus_open (test-bus-cleanup.c:42)
==6363==    by 0x112AAE: main (test-bus-cleanup.c:87)
==6363==
...
$ ./libtool --mode=execute valgrind ./test-bus-cleanup
==6584== LEAK SUMMARY:
...
==6584==      possibly lost: 10,566 bytes in 27 blocks

0 files changed, 0 insertions, 0 deletions