test-bus-util: add a simple test for bus_request_name_async_may_reload_dbus()

test-bus-util: add a simple test for bus_request_name_async_may_reload_dbus()

This shows a minor memleak:
==1883== 24 bytes in 1 blocks are definitely lost in loss record 1 of 1
==1883==    at 0x4C2DBAB: malloc (vg_replace_malloc.c:299)
==1883==    by 0x4E9D385: malloc_multiply (alloc-util.h:69)
==1883==    by 0x4EA2959: bus_request_name_async_may_reload_dbus (bus-util.c:1841)
==1883==    by ...

The exchange of messages is truncated at two different points: once right
after the first callback is requested, and the second time after the full
sequence has run (usually resulting in an error because of policy).