dbus_message_cache_or_finalize: allow message cache to be disabled at runtime

dbus_message_cache_or_finalize: allow message cache to be disabled at runtime

This should make it easier to diagnose message-related ref leaks,
use-after-free, etc. with Valgrind: for optimal results (and pessimal
performance), we want to avoid re-using memory blocks for as long as
possible.

For now this is conditional on DBUS_BUILD_TESTS. It could get its own
conditional if desired.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=37286
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Lennart Poettering <lennart@poettering.net>