Add a private "get mono_time" API to `tox_private.h` and use it in auto_tests.

[iphydf]

Make it so we don't need to rely on dangerous casts here:

c-toxcore/auto_tests/auto_test_support.c

Line 152 in e230ad4

Mono_Time *mono_time = ((Messenger *)autotox->tox)->mono_time;

The cast here relies on the fact that mono_time is at the same struct offset in struct Messenger and struct Tox.

[iphydf]

I think this is fixed, but I'll check tomorrow.