[Dbix-class] multiple has_many rels in prefetch not supported in 0.081?

[Peter Rabbitson]

Moritz Onken wrote:
> After talking to mst, the exception was degraded to a warn.
> Patch attached.
> 

Why lose the verbosity of the warning? How about:

carp (
    "Prefetching multiple has_many rels ${last} and ${pre} "
    .(length($as_prefix)
        ? "at the same level (${as_prefix}) "
        : "at top level "
    )
    . 'will currently disrupt both the functionality of $rs->count(), '
    . 'and the amount of objects retrievable via $rs->next(). '
    . 'Use at your own risk.'
);


Also don't blindly disable tests, but instead adjust them to test if the
proper warning is emitted.