Plug a mem leak in the gdbus-proxy test

Plug a mem leak in the gdbus-proxy test

==23341== 65 bytes in 3 blocks are definitely lost in loss record 927 of 1,020
==23341==    at 0x4005BDC: malloc (vg_replace_malloc.c:195)
==23341==    by 0x4057094: g_malloc (gmem.c:134)
==23341==    by 0x40573DB: g_malloc_n (gmem.c:281)
==23341==    by 0x40717FC: g_strdup (gstrfuncs.c:101)
==23341==    by 0x4147F56: value_lcopy_string (gvaluetypes.c:313)
==23341==    by 0x4123F0B: g_object_get_valist (gobject.c:1643)
==23341==    by 0x41240FF: g_object_get (gobject.c:1731)
==23341==    by 0x804C39E: test_basic (gdbus-proxy.c:522)

Bug #628331.