Tests: allow dbus-glib to be replaced with use of libdbus-internal

Tests: allow dbus-glib to be replaced with use of libdbus-internal

We only use dbus-glib for its main loop; within dbus, DBusLoop is
available as an alternative, although it isn't thread-safe and
isn't public API.

For tests that otherwise only use libdbus public API, it's desirable to
be able to avoid DBusLoop, so we can run them against an installed
libdbus as an integration test. However, if we don't have dbus-glib,
we're going to have to use an in-tree main loop, which might as well
be DBusLoop.

The major disadvantage of using dbus-glib is that it isn't safe to
link both dbus-1 and dbus-internal at the same time. This is awkward
for a future test case that wants to use _dbus_getsid() in dbus-daemon.c,
but only on Windows (fd.o #54445). If we use the same API wrapper around
both dbus-glib and DBusLoop, we can compile that test against dbus-glib
or against DBusLoop, depending on the platform.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=68852
Reviewed-by: Ralf Habacker <ralf.habacker@freenet.de>