Now that there is no code outside test/ that calls into this, we can
move it into test/, reducing the size of libdbus.

dbus-test-tap.[ch] still need to stay in dbus/ as long as there is
code in dbus/ or bus/ relying on them, and also need to be linked into
libdbus as long as there is other code in libdbus relying on them,
so they stay where they are for now. Similarly, dbus-tests.h lists
the tests that are still embedded in libdbus, and must stay where
it is for the moment.

With this move, various tests now need to be linked to the dbus-testutils
convenience library.

Signed-off-by: Simon McVittie <smcv@collabora.com>