nmtst_main_context_iterate_until() is a macro, and we don't want to restrict the
valid integer type (or range) of the "timeout_msec" argument.
In particular, if the user calculates a timeout with "timestamp_msec -
now_msec", the resulting "timeout_msec" might be a negative gint64.
We should handle that gracefully, and not let it be cast to a huge
unsigned int.