Add support for building "modular" tests, which require GLib and dbus-glib

Add support for building "modular" tests, which require GLib and dbus-glib

For the moment, the CMake build system only knows about the existing
"embedded tests"; make it define both symbols, though.

We use GLib because it has GTester (and life's too short to write yet another
JUnit clone), and dbus-glib for the main-loop integration only (see
fd.o #31515 for thoughts on incorporating just those two functions in a
separate library in the dbus tarball).

I'm not using DBusLoop for the main loop because I specifically don't
want to use non-public API or ABI of libdbus in the modular tests. If we make
sure they work against a shared libdbus, we can use them to test the
installed system, with "make installcheck".

Reviewed-by: Will Thompson <will.thompson@collabora.co.uk>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=34570