internals: Make a minimal _dbus_test_oom_handling() universally available

internals: Make a minimal _dbus_test_oom_handling() universally available

Previously, it was only available under DBUS_ENABLE_EMBEDDED_TESTS,
because the infrastructure to pretend malloc had failed is only
compiled then. However, I'd like to use it in more modular tests, to
avoid test-dbus continuing to grow. To facilitate that, inline a
trivial version of it when DBUS_ENABLE_EMBEDDED_TESTS is disabled:
it just calls the function, once, without doing any strange things to
the malloc interface.

Similarly, amend the stub implementation of
_dbus_get_malloc_blocks_outstanding() so that references to it are
syntactically valid, and move the DBusTestMemoryFunction typedef so
that it can be used with or without DBUS_ENABLE_EMBEDDED_TESTS.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=101568